The M-profile architecture floating point system supports
lazy FP state preservation, where FP registers are not
pushed to the stack when an exception occurs but are instead
only saved if and when the first FP instruction in the exception
handler is executed. Implement this in QEMU, corresponding
to the check of LSPACT in the pseudocode ExecuteFPCheck().
Backports commit e33cf0f8d8c9998a7616684f9d6aa0d181b88803 from qemu
Pushing registers to the stack for v7M needs to handle three cases:
* the "normal" case where we pend exceptions
* an "ignore faults" case where we set FSR bits but
do not pend exceptions (this is used when we are
handling some kinds of derived exception on exception entry)
* a "lazy FP stacking" case, where different FSR bits
are set and the exception is pended differently
Implement this by changing the existing flag argument that
tells us whether to ignore faults or not into an enum that
specifies which of the 3 modes we should handle.
Backports commit a356dacf647506bccdf8ecd23574246a8bf615ac from qemu
In the v7M architecture, if an exception is generated in the process
of doing the lazy stacking of FP registers, the handling of
possible escalation to HardFault is treated differently to the normal
approach: it works based on the saved information about exception
readiness that was stored in the FPCCR when the stack frame was
created. Provide a new function armv7m_nvic_set_pending_lazyfp()
which pends exceptions during lazy stacking, and implements
this logic.
This corresponds to the pseudocode TakePreserveFPException().
Backports the relevant parts of commit
a99ba8ab1601904e0fa20325192fc850362ce80e from qemu
Add a new helper function which returns the MMU index to use
for v7M, where the caller specifies all of the security
state, privilege level and whether the execution priority
is negative, and reimplement the existing
arm_v7m_mmu_idx_for_secstate_and_priv() in terms of it.
We are going to need this for the lazy-FP-stacking code.
Backports commit fa6252a988dbe440cd6087bf93cbe0887f0c401b from qemu
The M-profile FPCCR.ASPEN bit indicates that automatic floating-point
context preservation is enabled. Before executing any floating-point
instruction, if FPCCR.ASPEN is set and the CONTROL FPCA/SFPA bits
indicate that there is no active floating point context then we
must create a new context (by initializing FPSCR and setting
FPCA/SFPA to indicate that the context is now active). In the
pseudocode this is handled by ExecuteFPCheck().
Implement this with a new TB flag which tracks whether we
need to create a new FP context.
Backports commit 6000531e19964756673a5f4b694a649ef883605a from qemu
The M-profile FPCCR.S bit indicates the security status of
the floating point context. In the pseudocode ExecuteFPCheck()
function it is unconditionally set to match the current
security state whenever a floating point instruction is
executed.
Implement this by adding a new TB flag which tracks whether
FPCCR.S is different from the current security state, so
that we only need to emit the code to update it in the
less-common case when it is not already set correctly.
Note that we will add the handling for the other work done
by ExecuteFPCheck() in later commits.
Backports commit 6d60c67a1a03be32c3342aff6604cdc5095088d1 from qemu
We are close to running out of TB flags for AArch32; we could
start using the cs_base word, but before we do that we can
economise on our usage by sharing the same bits for the VFP
VECSTRIDE field and the XScale XSCALE_CPAR field. This
works because no XScale CPU ever had VFP.
Backports commit ea7ac69d124c94c6e5579145e727adec9ccbefef from qemu
Move the NS TBFLAG down from bit 19 to bit 6, which has not
been used since commit c1e3781090b9d36c60 in 2015, when we
started passing the entire MMU index in the TB flags rather
than just a 'privilege level' bit.
This rearrangement is not strictly necessary, but means that
we can put M-profile-only bits next to each other rather
than scattered across the flag word.
Backports commit 7fbb535f7aeb22896fedfcf18a1eeff48165f1d7 from qemu
Handle floating point registers in exception return.
This corresponds to pseudocode functions ValidateExceptionReturn(),
ExceptionReturn(), PopStack() and ConsumeExcStackFrame().
Backports commit 6808c4d2d2826920087533f517472c09edc7b0d2 from qemu
The magic value pushed onto the callee stack as an integrity
check is different if floating point is present.
Backports commit 0dc51d66fcfcc4c72011cdafb401fd876ca216e7 from qemu
The TailChain() pseudocode specifies that a tail chaining
exception should sanitize the excReturn all-ones bits and
(if there is no FPU) the excReturn FType bits; we weren't
doing this.
Backports commit 60fba59a2f9a092a44b688df5d058cdd6dd9c276 from qemu
For v8M floating point support, transitions from Secure
to Non-secure state via BLNS and BLXNS must clear the
CONTROL.SFPA bit. (This corresponds to the pseudocode
BranchToNS() function.)
Backports commit 3cd6726f0ba7cc77342ee721bd86094e13b2a42a from qemu
Implement the code which updates the FPCCR register on an
exception entry where we are going to use lazy FP stacking.
We have to defer to the NVIC to determine whether the
various exceptions are currently ready or not.
Backports commit b593c2b81287040ab6f452afec6281e2f7ee487b from qemu
Handle floating point registers in exception entry.
This corresponds to the FP-specific parts of the pseudocode
functions ActivateException() and PushStack().
We defer the code corresponding to UpdateFPCCR() to a later patch.
Backports commit 0ed377a8013f40653a83f6ad2c9693897522d7dc from qemu
Currently the code in v7m_push_stack() which detects a violation
of the v8M stack limit simply returns early if it does so. This
is OK for the current integer-only code, but won't work for the
floating point handling we're about to add. We need to continue
executing the rest of the function so that we check for other
exceptions like not having permission to use the FPU and so
that we correctly set the FPCCR state if we are doing lazy
stacking. Refactor to avoid the early return.
Backports commit 3432c79a4e7345818d2defcf9e61a1bcb2907f9f from qemu
The M-profile CONTROL register has two bits -- SFPA and FPCA --
which relate to floating-point support, and should be RES0 otherwise.
Handle them correctly in the MSR/MRS register access code.
Neither is banked between security states, so they are stored
in v7m.control[M_REG_S] regardless of current security state.
Backports commit 2e1c5bcd32014c9ede1b604ae6c2c653de17fc53 from qemu
If the floating point extension is present, then the SG instruction
must clear the CONTROL_S.SFPA bit. Implement this.
(On a no-FPU system the bit will always be zero, so we don't need
to make the clearing of the bit conditional on ARM_FEATURE_VFP.)
Backports commit 1702071302934af77a072b7ee7c5eadc45b37573 from qemu
Correct the decode of the M-profile "coprocessor and
floating-point instructions" space:
* op0 == 0b11 is always unallocated
* if the CPU has an FPU then all insns with op1 == 0b101
are floating point and go to disas_vfp_insn()
For the moment we leave VLLDM and VLSTM as NOPs; in
a later commit we will fill in the proper implementation
for the case where an FPU is present.
Backports commit 8859ba3c9625e7ceb5599f457a344bcd7c5e112b from qemu
Like AArch64, M-profile floating point has no FPEXC enable
bit to gate floating point; so always set the VFPEN TB flag.
M-profile also has CPACR and NSACR similar to A-profile;
they behave slightly differently:
* the CPACR is banked between Secure and Non-Secure
* if the NSACR forces a trap then this is taken to
the Secure state, not the Non-Secure state
Honour the CPACR and NSACR settings. The NSACR handling
requires us to borrow the exception.target_el field
(usually meaningless for M profile) to distinguish the
NOCP UsageFault taken to Secure state from the more
usual fault taken to the current security state.
Backports commit d87513c0abcbcd856f8e1dee2f2d18903b2c3ea2 from qemu
The only "system register" that M-profile floating point exposes
via the VMRS/VMRS instructions is FPSCR, and it does not have
the odd special case for rd==15. Add a check to ensure we only
expose FPSCR.
Backports commit ef9aae2522c22c05df17dd898099dd5c3f20d688 from qemu
The M-profile floating point support has three associated config
registers: FPCAR, FPCCR and FPDSCR. It also makes the registers
CPACR and NSACR have behaviour other than reads-as-zero.
Add support for all of these as simple reads-as-written registers.
We will hook up actual functionality later.
The main complexity here is handling the FPCCR register, which
has a mix of banked and unbanked bits.
Note that we don't share storage with the A-profile
cpu->cp15.nsacr and cpu->cp15.cpacr_el1, though the behaviour
is quite similar, for two reasons:
* the M profile CPACR is banked between security states
* it preserves the invariant that M profile uses no state
inside the cp15 substruct
Backports commit d33abe82c7c9847284a23e575e1078cccab540b5 from qemu
Enforce that for M-profile various FPSCR bits which are RES0 there
but have defined meanings on A-profile are never settable. This
ensures that M-profile code can't enable the A-profile behaviour
(notably vector length/stride handling) by accident.
Backports commit 5bcf8ed9401e62c73158ba110864ee1375558bf7 from qemu
This change adapts io_readx() to its input access_type. Currently
io_readx() treats any memory access as a read, although it has an
input argument "MMUAccessType access_type". This results in:
1) Calling the tlb_fill() only with MMU_DATA_LOAD
2) Considering only entry->addr_read as the tlb_addr
Buglink: https://bugs.launchpad.net/qemu/+bug/1825359
Backports commit ef5dae6805cce7b59d129d801bdc5db71bcbd60d from qemu
This will not necessarily restrict the size of the TB, since for v7
the majority of constant pool usage is for calls from the out-of-line
ldst code, which is already at the end of the TB. But this does
allow us to save one insn per reference on the off-chance.
Backports commit b4b82d7e9caff7ccca5c621817b5a4b8e95eb9b1 from qemu
There is no point in coding for a 2GB offset when the max TB size
is already limited to 64k. If we further restrict to 32k then we
can eliminate the extra ADDIS instruction.
Backports commit a7cdaf710f2aaaf0be855a338dd67463d4bb99e2 from qemu
If the TB generates too much code, such that backend relocations
overflow, try again with a smaller TB. In support of this, move
relocation processing from a random place within tcg_out_op, in
the handling of branch opcodes, to a new function at the end of
tcg_gen_code.
This is not a complete solution, as there are additional relocs
generated for out-of-line ldst handling and constant pools.
Backports commit 7ecd02a06f8f4c0bbf872ecc15e37035b7e1df5f from qemu
If a TB generates too much code, try again with fewer insns.
Fixes: https://bugs.launchpad.net/bugs/1824853
Backports commit 6e6c4efed995d9eca6ae0cfdb2252df830262f50 from qemu
In order to handle TB's that translate to too much code, we
need to place the control of the length of the translation
in the hands of the code gen master loop.
Backports commit 8b86d6d25807e13a63ab6ea879f976b9f18cc45a from qemu
Will be helpful for s390x. Input 128 bit and output 64 bit only,
which is sufficient for now.
Backports commit 2089fcc9e7b4174d1c351eaa7d277c02188a6dd2 from qemu
Add a new base CPU model called 'Dhyana' to model processors from Hygon
Dhyana(family 18h), which derived from AMD EPYC(family 17h).
The following features bits have been removed compare to AMD EPYC:
aes, pclmulqdq, sha_ni
The Hygon Dhyana support to KVM in Linux is already accepted upstream[1].
So add Hygon Dhyana support to Qemu is necessary to create Hygon's own
CPU model.
Reference:
[1] https://git.kernel.org/tip/fec98069fb72fb656304a3e52265e0c2fc9adf87
Backports commit 8d031cec366f26669807eb43f61eb335973b7053 from qemu
Not all targets define a full set of suffix strings for the
NB_MMU_MODES that they have. In this situation, don't define any
helper functions for that mode, rather than defining helper functions
with no suffix at all. The MMU mode is still functional; it is merely
not directly accessible via cpu_ld*_MODE from target helper functions.
Also add an "NB_MMU_MODES >= 2" check to the definition of the mode 1
helpers -- some targets only define one MMU mode.
Backports commit de5ee4a888667ca0a198f0743d70075d70564117 from qemu
Add documentation of what the cpu_*_* accessors look like.
Correct some minor errors in the existing documentation of the
direct _p accessor family. Remove the near-duplicate comment
on the _p accessors from cpu-all.h and replace it with a reference
to the comment in bswap.h.
Backports commit db5fd8d709fd57f4d4f11edfca9f421f657f4508 from qemu
The cpu_ldfq/stfq/ldfl/stfl accessors for loading and storing
float32 and float64 are completely unused, so delete them.
(The union they use for converting from the float32/float64
type to uint32_t or uint64_t is the wrong way to do it anyway:
they should be using make_float* and float*_val.)
Backports commit 82f11917c99e3c7fa3d6aa98572ecc98c7324c2f from qemu
The _raw macros and their helpers saddr() and laddr() are now
totally unused -- delete them.
Backports commit 800e2ecc896beb6b79e7333c762da163b6a9135a from qemu
The ld*_raw and st*_raw macros are now only used within the code
produced by cpu_ldst_template.h, and only in three places.
Expand these out to just call the ld_p and st_p functions directly.
Note that in all the callsites the address argument is a uintptr_t,
so we can drop that part of the double-cast used in the saddr() and
laddr() macros.
Backports commit 355392329e4a843580e53cb027ed85e0cbebb640 from qemu
Use inline functions rather than macros for cpu_ld/st accessors
for the *-user configurations, as we already do for softmmu.
This has a two advantages:
* we can actually typecheck our arguments
* we don't need to leak the _raw macros everywhere
Since the _kernel functions were only used by target-i386/seg_helper.c,
put the definitions for them in that file too. (It already has the
similar template include code to define them for the softmmu case,
so it makes sense to have it deal with defining them for user-only.)
Backports commit 9220fe54c679d145232a28df6255e166ebf91bab from qemu
Prevents an infinite loop case if mapping near the upper boundary of an
address space on 32-bit emulated targets. i.e. mapping at 0xFFFFF000
with a size of 4096 won't overflow back to zero.
While we're at it, also tidy up the unicorn-specific functions.
This wasn't subtracting the size of the instruction off the PC like how
the ARM mode tracing was performing the tracing. This simplifies it and
makes the behavior identical.
Allows non-AArch64 environments to always access coprocessors initially.
Removes the need to do avoidable register management when testing
floating-point code.
In the accessor functions ld*_he_p() and st*_he_p() we use memcpy()
to perform a load or store to a pointer which might not be aligned
for the size of the type. We rely on the compiler to optimize this
memcpy() into an efficient load or store instruction where possible.
This is required for good performance, but at the moment it is also
required for correct operation, because some users of these functions
require that the access is atomic if the pointer is aligned, which
will only be the case if the compiler has optimized out the memcpy().
(The particular example where we discovered this is the virtio
vring_avail_idx() which calls virtio_lduw_phys_cached() which
eventually ends up calling lduw_he_p().)
Unfortunately some compile environments, such as the fortify-source
setup used in Alpine Linux, define memcpy() to a wrapper function
in a way that inhibits this compiler optimization.
The correct long-term fix here is to add a set of functions for
doing atomic accesses into AddressSpaces (and to other relevant
families of accessor functions like the virtio_*_phys_cached()
ones), and make sure that callsites which want atomic behaviour
use the correct functions.
In the meantime, switch to using __builtin_memcpy() in the
bswap.h accessor functions. This will make us robust against things
like this fortify library in the short term. In the longer term
it will mean that we don't end up with these functions being really
badly-performing even if the semantics of the out-of-line memcpy()
are correct.
Fix a TCG crash due to attempting an atomic increment
operation without having set up the address first.
This is a similar case to that dealt with in commit
e84fcd7f662a0d8198703, and we fix it in the same way.
Fixes: https://bugs.launchpad.net/qemu/+bug/1807675
Backports commit 8cb2ca3d7479748587313f0b34034a3f8aa08c92 from qemu
This ensures that softmmu directories are culled after a
"./configure --target-list=x86_64-linux-user".
Backports commit b7c11e574977a0addfbbdb89377c6f52affe64ec from qemu
Currently, a callback registered through the RAMBlock notifier
is not able to get the memory region type (i.e callback is not
able to use memory_region_is_ram_device function). This is
because mr->ram assignment happens _after_ the memory is allocated
whereas the callback is executed during allocation.
Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1667249
Backports commit 2ddb89b00f947f785c9ca6742f28f954e3b75e62 from qemu
While running the GCC test suite against 4.0.0-rc0, Kito found a
regression introduced by the decodetree conversion that caused divuw and
remuw to sign-extend their inputs. The ISA manual says they are
supposed to be zero extended:
DIVW and DIVUW instructions are only valid for RV64, and divide the
lower 32 bits of rs1 by the lower 32 bits of rs2, treating them as
signed and unsigned integers respectively, placing the 32-bit
quotient in rd, sign-extended to 64 bits. REMW and REMUW
instructions are only valid for RV64, and provide the corresponding
signed and unsigned remainder operations respectively. Both REMW
and REMUW always sign-extend the 32-bit result to 64 bits, including
on a divide by zero.
Here's Kito's reduced test case from the GCC test suite
unsigned calc_mp(unsigned mod)
{
unsigned a,b,c;
c=-1;
a=c/mod;
b=0-a*mod;
if (b > mod) { a += 1; b-=mod; }
return b;
}
int main(int argc, char *argv[])
{
unsigned x = 1234;
unsigned y = calc_mp(x);
if ((sizeof (y) == 4 && y != 680)
|| (sizeof (y) == 2 && y != 134))
abort ();
exit (0);
}
I haven't done any other testing on this, but it does fix the test case.
Backports commit f17e02cd3731bdfe2942d1d0b2a92f26da02408c from qemu
cortex-a7 and cortex-a15 have pmus (PMUv2) and they advertise
them in ID_DFR0. Let's allow them to function. This also enables
the pmu cpu property to work with these cpu types, i.e. we can
now do '-cpu cortex-a15,pmu=off' to remove the pmu.
Backports commit a46118fc16537a593119e5b316052a98514046bb from qemu
Fix a QEMU NULL derefence that occurs when the guest attempts to
enable PMU counters with a non-v8 cpu model or a v8 cpu model
which has not configured a PMU.
Backports commit cbbb3041fe2f57a475cef5d6b0ef836118aad106 from qemu
The second word has been loaded from the unincremented
address since the first commit.
Backports commit a036f5302c13634f3d375615b2949fd1fa1657b6 from qemu
Currently, the Cascadelake-Server, Icelake-Client, and
Icelake-Server are always generating the following warning:
qemu-system-x86_64: warning: \
host doesn't support requested feature: CPUID.07H:ECX [bit 4]
This happens because OSPKE was never returned by
GET_SUPPORTED_CPUID or x86_cpu_get_supported_feature_word().
OSPKE is a runtime flag automatically set by the KVM module or by
TCG code, was always cleared by x86_cpu_filter_features(), and
was not supposed to appear on the CPU model table.
Remove the OSPKE flag from the CPU model table entries, to avoid
the bogus warning and avoid returning invalid feature data on
query-cpu-* QMP commands. As OSPKE was always cleared by
x86_cpu_filter_features(), this won't have any guest-visible
impact.
Include a test case that should detect the problem if we introduce
a similar bug again.
Fixes: c7a88b52f62b ("i386: Add new model of Cascadelake-Server")
Fixes: 8a11c62da914 ("i386: Add new CPU model Icelake-{Server,Client}")
Backports commit bb4928c7cafe50ab2137a0034e350ef1bfa044d9 from qemu
Now that kvm_arch_get_supported_cpuid() will only return
arch_capabilities if QEMU is able to initialize the MSR properly,
we know that the feature is safely migratable.
Backports commit 014018e19b3c54dd1bf5072bc912ceffea40abe8 from qemu
If vectored interrupts are enabled (bits[1:0]
of mtvec/stvec == 1) then use the following
logic for trap entry address calculation:
pc = mtvec + cause * 4
In addition to adding support for vectored interrupts
this patch simplifies the interrupt delivery logic
by making sync/async cause decoding and encoding
steps distinct.
The cause code and the sign bit indicating sync/async
is split at the beginning of the function and fixed
cause is renamed to cause. The MSB setting for async
traps is delayed until setting mcause/scause to allow
redundant variables to be eliminated. Some variables
are renamed for conciseness and moved so that decls
are at the start of the block.
Backports commit acbbb94e5730c9808830938e869d243014e2923a from qemu
This effectively changes riscv_cpu_update_mip
from edge to level. i.e. cpu_interrupt or
cpu_reset_interrupt are called regardless of
the current interrupt level.
Fixes WFI doesn't return when a IPI is issued:
- https://github.com/riscv/riscv-qemu/issues/132
To test:
1) Apply RISC-V Linux CPU hotplug patch:
- http://lists.infradead.org/pipermail/linux-riscv/2018-May/000603.html
2) Enable CONFIG_CPU_HOTPLUG in linux .config
3) Try to offline and online cpus:
echo 1 > /sys/devices/system/cpu/cpu2/online
echo 0 > /sys/devices/system/cpu/cpu2/online
echo 1 > /sys/devices/system/cpu/cpu2/online
Backports commit d26f5a423438e579d3ff0ca35e44edb966a36233 from qemu
This change checks elf_flags for EF_RISCV_RVE and if
present uses the RVE linux syscall ABI which uses t0
for the syscall number instead of a7.
Warn and exit if a non-RVE ABI binary is run on a
cpu with the RVE extension as it is incompatible.
Backports relevant parts of 5836c3eccedb6dfab16b8f606f2de24b8938b69c
from qemu
We can't allow the supervisor to control SEIP as this would allow the
supervisor to clear a pending external interrupt which will result in
lost a interrupt in the case a PLIC is attached. The SEIP bit must be
hardware controlled when a PLIC is attached.
This logic was previously hard-coded so SEIP was always masked even
if no PLIC was attached. This patch adds riscv_cpu_claim_interrupts
so that the PLIC can register control of SEIP. In the case of models
without a PLIC (spike), the SEIP bit remains software controlled.
This interface allows for hardware control of supervisor timer and
software interrupts by other interrupt controller models.
Backports commit e3e7039cc24ecf47d81c091e8bb04552d6564ad8 from qemu
Add a debugger field to CPURISCVState. Add riscv_csrrw_debug function
to set it. Disable mode checks when debugger field true.
Backports commit 753e3fe207db08ce0ef0405e8452c3397c9b9308 from qemu
This adds some missing CSR_* register macros, and documents some as being
priv v1.9.1 specific.
Backports commit 8e73df6aa3f2f0e5c26c03a94a88406616291815 from qemu
during the refactor to decodetree we removed the manual decoding that is
necessary for c.jal/c.addiw and removed the translation of c.flw/c.ld
and c.fsw/c.sd. This reintroduces the manual parsing and the
omited implementation.
Backports commit f330433b3633647b047cfa418c2ca4d18fda69c7 from qemu
These instructions do not trap when SVE is disabled in EL0,
causing them to be executed with wrong size information.
Backports commit 5de56742a3c91de3d646326bec43a989bba83ca4 from qemu
Some generic arch timer registers are Config-RW in the EL0,
which means the EL0 exception level can have write permission
if it is appropriately configured.
When VM access registers, QEMU firstly checks whether they have RW
permission, then check whether it is appropriately configured.
If they are defined to read only in EL0, even though they have been
appropriately configured, they still do not have write permission.
So need to add the write permission according to ARMV8 spec when
define it.
Backports commit daf1dc5f82cefe2a57f184d5053e8b274ad2ba9a from qemu
with all 16 bit insns moved to decodetree no path is falling back to
gen_system(), so we can remove it.
Backports commit 8f7bc273868939f0821e07fb23792db63d45bffb from qemu
manual decoding in gen_arith() is not necessary with decodetree. For now
the function is called trans_arith as the original gen_arith still
exists. The former will be renamed to gen_arith as soon as the old
gen_arith can be removed.
Backports commit f2ab1728675772cd475a33f4df3d2f68a22c188f from qemu
gen_arith_imm() does a lot of decoding manually, which was hard to read
in case of the shift instructions and is not necessary anymore with
decodetree.
Backports commit 7a50d3e2ae7f13b24fe55990ea0b8ddcbbb43130 from qemu
With decodetree we don't need to convert RISC-V opcodes into to MemOps
as the old gen_store() did.
Backports commit bce8a342a1f0919479d18ec812b100136daa746b from qemu
With decodetree we don't need to convert RISC-V opcodes into to MemOps
as the old gen_load() did.
Backports commit 98898b20e9cca462843c22ad952c216ffd57d654 from qemu
We now utilizes argument-sets of decodetree such that no manual
decoding is necessary.
Backports commit 090cc2c898a04e42350eabf1bcf7d245471603f9 from qemu
we cannot remove the call to gen_arith() in decode_RV32_64G() since it
is used to translate multiply instructions.
Backports commit b73a987b09ad5081123dc6b1e8e6c8305a1c8673 from qemu
this splits the 64-bit only instructions into its own decode file such
that we generate the decoder for these instructions only for the RISC-V
64 bit target.
Backports commit 7e45a682edc32ba90d6955215f062210531b835b from qemu
for now only LUI & AUIPC are decoded and translated. If decodetree fails, we
fall back to the old decoder.
Backports commit 2a53cff418335ccb4719e9a94fde55f6ebcc895d from qemu
Previously this would result in an exception for shifting
the field mask by a negative number.
Backports commit 2decfc95583dc28add69810eaca6ada7b4b44d3a from qemu
This is interesting for bisection, where an output file is plumbed,
but does not yet have patterns.
Backports commit 82bfac1c06cadeb5c7252734dc695d951185916c from qemu
As a consequence, the 'return false' gets pushed up one level.
This will allow us to perform some other action when the
translator returns failure.
Backports commit eb6b87fac70dd62e3f1286703db20c012e7a9611 from qemu
