PIE is supported on many other hosts besides x86.
The default for non-x86 is now the same as x86: pie is used
if supported, and may be forced via --enable/--disable-pie.
The original commit (40d6444e91c) said:
"Non-x86 are not changed, as they require TCG changes"
but I think that's wrong -- there's nothing about PIE that
affects TCG one way or another.
Tested on aarch64 (bionic) and ppc64le (centos 7) hosts.
Backports commit d2cd29e30736afd4a1e8cac3cf4da360bbc65978 from qemu
The comment in tcg-runtime-gvec.c about CONFIG_VECTOR16 says that
tcg-op-gvec.c has eliminated size 8 vectors, and only passes on
multiples of 16. This may have been true of the first few operations,
but is not true of all operations.
In particular, multiply, shift by scalar, and compare of 8- and 16-bit
elements are not expanded inline if host vector operations are not
supported.
For an x86_64 host that does not support AVX, this means that we will
fall back to the helper, which will attempt to use SSE instructions,
which will SEGV on an invalid 8-byte aligned memory operation.
This patch simply removes the CONFIG_VECTOR16 code and configuration
without further simplification.
Buglink: https://bugs.launchpad.net/bugs/1863508
Backports commit 43d1ccd2a02fadf3c36b46a8c31125a28864d141 from qemu
Python 3.5 is the oldest Python version available on our
supported build platforms, and Python 2 end of life will be 3
weeks after the planned release date of QEMU 4.2.0. Drop Python
2 support from configure completely, and require Python 3.5 or
newer.
Backports commit ddf90699631db53c981b6a5a63d31c08e0eaeec7 from qemu
This is a left-over from commit
c12b6d70e384c769ca372e15ffd19b3e9f563662 ("pixman: drop submodule")
Backports commit fbb04e760f4818a1ba9cfde0a2571a15cd4f49f7 from qemu
9b9c37c364 ("tcg-sparc: Assume v9 cpu always, i.e. force v8plus in
32-bit mode.", 2012-09-21) removed the need for this variable and
most of the references to it, but this one.
Remove defunct code, no effect or functionality change expected.
Backports commit edd318b7a3c54321ead363ffc1e22ebac5785f1d from qemu
This patch moves the define of target access alignment earlier from
target/foo/cpu.h to configure.
Suggested in Richard Henderson's reply to "[PATCH 1/4] tcg: TCGMemOp is now
accelerator independent MemOp"
Backports commit 52bf9771fdfce98e98cea36a17a18915be6f6b7f from qemu
The configure script breaks when the qemu source directory is in a path
containing white spaces, in particular the list of targets is not
correctly generated when calling "./configure --help" because of how the
default_target_list variable is built.
In addition to that, *building* qemu from a directory with spaces breaks
some assumptions in the Makefiles, even if the original source path does
not contain spaces like in the case of an out-of-tree build, or when
symlinks are involved.
To avoid these issues, refuse to run the configure script and the
Makefile if there are spaces or colons in the source path or the build
path, taking as inspiration what the kbuild system in linux does.
Buglink: https://bugs.launchpad.net/qemu/+bug/1817345
Backports commit 4ace32e22713ffd79deb221ae0134652c7c15428 from qemu
Unless overridden via an env var or configure arg, QEMU will only look
for the 'python' binary in $PATH. This is unhelpful on distros which
are only shipping Python 3.x (eg Fedora) in their default install as,
if they comply with PEP 394, the bare 'python' binary won't exist.
This changes configure so that by default it will search for all three
common python binaries, preferring to find Python 3.x versions.
Backports commit faf441429adfe5767be52c5dcdb8bc03161d064f from qemu
When running "make" in a build directory from the pre-Kconfig merge time,
the build process currently fails with:
make: *** No rule to make target `.../default-configs/pci.mak',
needed by `aarch64-softmmu/config-devices.mak'. Stop.
To make sure that this problem at least goes away when the user runs
"configure" (or "sh config.status") again, we have to make sure that
we re-generate the .mak.d files. Thus remove the old stale files
while running the configure script.
Backports commit 9c79024225af6b3ae04ea2dd94a5e5c4132a9e65 from qemu
Without the -Wno-typedef-redefinition option, clang complains if a typedef
gets redefined in gnu99 mode (since this is officially a C11 feature). This
used to also happen with older versions of GCC, but since we've bumped our
minimum GCC version to 4.8, all versions of GCC that we support do not seem
to issue this warning in gnu99 mode anymore. So this has become a common
problem for people who only test their code with GCC - they do not notice
the issue until they submit their patches and suddenly patchew or a
maintainer complains.
Now that we do not urgently need to keep the code clean from typedef
redefintions anymore with recent versions of GCC, we can ease the
situation with clang, too, and simply shut these warnings off for good.
Backports commit e6e90feedb706b1b92827a5977b37e1e8defb8ef from qemu
Since OpenBSD 6.0 [1], W^X is enforced by default [2].
TCG requires WX access. Disable W^X if it is available.
This fixes:
\# lm32-softmmu/qemu-system-lm32
Could not allocate dynamic translator buffer
\# sysctl kern.wxabort=1
kern.wxabort: 0 -> 1
\# lm32-softmmu/qemu-system-lm32
mmap: Not supported
Abort trap (core dumped)
\# gdb -q lm32-softmmu/qemu-system-lm32 qemu-system-lm32.core
(gdb) bt
\#0 0x000017e3c156c50a in _thread_sys___syscall () at {standard input}:5
\#1 0x000017e3c15e5d7a in *_libc_mmap (addr=Variable "addr" is not available.) at /usr/src/lib/libc/sys/mmap.c:47
\#2 0x000017e17d9abc8b in alloc_code_gen_buffer () at /usr/src/qemu/accel/tcg/translate-all.c:1064
\#3 0x000017e17d9abd04 in code_gen_alloc (tb_size=0) at /usr/src/qemu/accel/tcg/translate-all.c:1112
\#4 0x000017e17d9abe81 in tcg_exec_init (tb_size=0) at /usr/src/qemu/accel/tcg/translate-all.c:1149
\#5 0x000017e17d9897e9 in tcg_init (ms=0x17e45e456800) at /usr/src/qemu/accel/tcg/tcg-all.c:66
\#6 0x000017e17d9891b8 in accel_init_machine (acc=0x17e3c3f50800, ms=0x17e45e456800) at /usr/src/qemu/accel/accel.c:63
\#7 0x000017e17d989312 in configure_accelerator (ms=0x17e45e456800, progname=0x7f7fffff07b0 "lm32-softmmu/qemu-system-lm32") at /usr/src/qemu/accel/accel.c:111
\#8 0x000017e17d9d8616 in main (argc=1, argv=0x7f7fffff06b8, envp=0x7f7fffff06c8) at vl.c:4325
[1] https://www.openbsd.org/faq/upgrade60.html
[2] https://undeadly.org/cgi?action=article&sid=20160527203200
Backports commit 7776ea6b49873ed18a2111e25ed8a6d94bd73db8 from qemu
This ports over the RISC-V architecture from Qemu. This is currently a
very barebones transition. No code hooking or any fancy stuff.
Currently, you can feed it instructions and query the CPU state itself.
This also allows choosing whether or not RISC-V 32-bit or RISC-V 64-bit
is desirable through Unicorn's interface as well.
Extremely basic examples of executing a single instruction have been
added to the samples directory to help demonstrate how to use the basic
functionality.
Different versions of GCC and Clang use different versions of the C standard.
This repeatedly caused problems already, e.g. with duplicated typedefs:
https://lists.gnu.org/archive/html/qemu-devel/2018-11/msg05829.html
or with for-loop variable initializers:
https://lists.gnu.org/archive/html/qemu-devel/2019-01/msg00237.html
To avoid these problems, we should enforce the C language version to the
same level for all compilers. Since our minimum compiler versions is
GCC v4.8, our best option is "gnu99" for C code right now ("gnu17" is not
available there yet, and "gnu11" is marked as "experimental"), and "gnu++98"
for the few C++ code that we have in the repository.
Backports commit 7be41675f7cb16be7c8d2554add7a63fa43781a8 from qemu
Some functionality is dependent on the Python version
detected/configured on configure. While it's possible to run the
Python version later and check for the version, doing it once is
preferable. Also, it's a relevant information to keep in build logs,
as the overall behavior of the build can be affected by it.
Backports commit 755ee70ff758584b8b6190b2cab4b480402af201 from qemu
This removes some clutter in compilation logging, and allows some
easier tweaking per compilation unit/CFLAGS overriding.
Note that we can't move those define in os-win32.h, since they must be
set before the first system headers are included.
Backports commit 007e722c349839f430f10639ba8c94fe43acfe50 from qemu
Now that we require at least GCC 4.8, we don't need this als workaround
for 4.6 and 4.7 anymore.
Backports commit d376e9de07e9541a040609d30b73403c33b6afe0 from qemu
Since we have got a check for Clang >= 3.4 now, we do not need to
check for older Clang versions in the configure test for 128-bit ints
anymore.
Backports commit 40f87e2680a3fda6117a08db9106a01345806a66 from qemu
So far we only had implicit requirements for the minimum compiler version,
e.g. we require at least GCC 4.1 for the support of atomics. However,
such old compiler versions are not tested anymore by the developers, so
they are not really supported anymore. Since we recently declared explicitly
what platforms we intend to support, we can also get more explicit on the
compiler version now. The supported distributions use the following version
of GCC:
RHEL-7: 4.8.5
Debian (Stretch): 6.3.0
Debian (Jessie): 4.8.4
OpenBSD (ports): 4.9.4
FreeBSD (ports): 8.2.0
OpenSUSE Leap 15: 7.3.1
Ubuntu (Xenial): 5.3.1
macOS (Homebrew): 8.2.0
So we can safely assume GCC 4.8 these days. For Clang, the situation is
a little bit more ambiguous, since it is sometimes not available in the
main distros but rather third party repositories. At least Debian Jessie
uses version 3.5, and EPEL7 for RHEL7 uses 3.4, so let's use 3.4 as
minimum Clang version now - we still can adjust this later if necessary.
Unfortunately Apple uses different version numbers for the Clang that is
included in their Xcode suite, so we need to check the version numbers
for Xcode separately. Xcode 5.1 seems to be the first one that has been
shipped with LLVM 3.4, so use this version as the minimum there.
Backports commit efc6c070acabd92283f8786cc835684d09def12f from qemu
GCC7+ will no longer advertise support for 16-byte __atomic operations
if only cmpxchg is supported, as for x86_64. Fortunately, x86_64 still
has support for __sync_compare_and_swap_16 and we can make use of that.
AArch64 does not have, nor ever has had such support, so open-code it.
Backports commit e6cd4bb59b8154fa00da611200beef7eb4e8ec56 from qemu
The config.status script is auto-generated by configure upon
completion. The intention is that config.status can be later invoked by
the developer directly, or by make indirectly, to re-detect the same
environment that configure originally used.
The current config.status script, however, only contains a record of the
command line arguments to configure. Various environment variables have
an effect on what configure will find. In particular PKG_CONFIG_LIBDIR &
PKG_CONFIG_PATH vars will affect what libraries pkg-config finds. The
PATH var will affect what toolchain binaries and XXXX-config scripts are
found. The LD_LIBRARY_PATH var will affect what libraries are
found. Most commands have env variables that will override the name/path
of the default version configure finds.
All these key env variables should be recorded in the config.status script.
Autoconf would also preserve CFLAGS, LDFLAGS, LIBS, CPPFLAGS, but QEMU
deals with those differently, expecting extra flags to be set using
configure args, rather than env variables. At the end of the script we
also don't have the original values of those env vars, as we modify them
during configure.
Backports commit e811da7fe229cc17d98b230bdfeaf6d0631ea987 from qemu
This reverts commit 208ecb3e1acc8d55dab49fdf721a86d513691688. This was
causing problems by making DEF_TARGET_LIST pointless and having to
jump through hoops to build on mingw with a dully enabled config.
This includes a change to fix the per-guest TCG test probe which was
added after 208ecb3 and used TARGET_LIST.
Backports commit 2b1f35b9a85cf0232615a67e7ff523137a58795e from qemu
To be more accurate on its purpose and make code that looks for a certain
target out of this variable more readable.
Backports commit 208ecb3e1acc8d55dab49fdf721a86d513691688 from qemu
configure tries to detect if the compiler
supports 16-byte vector operations.
As stated in the comment of the detection
program, there is a problem with the system
compiler on GCC on Centos 7.
This program doesn't actually detect the problem
with GCC on RHEL7 on PPC64LE (Red Hat 4.8.5-28).
This patch updates the test to look more like
it is in QEMU helpers, and now detects the problem.
The error reported is:
CC ppc64-softmmu/accel/tcg/tcg-runtime-gvec.o
..//accel/tcg/tcg-runtime-gvec.c: In function ‘helper_gvec_shl8i’:
../accel/tcg/tcg-runtime-gvec.c:558:26: internal compiler error: in emit_move_insn, at expr.c:3495
*(vec8 *)(d + i) = *(vec8 *)(a + i) << shift;
^
Fixes: db43267 "tcg: Add generic vector expanders"
Backports commit 74912f6dad207bfdd5bfa8c6012c364ea9405fc7 from qemu
QEMU_BUILD_BUG_ON should use C11's _Static_assert, if the compiler supports it,
to provide more readable messages on failure.
We check for _Static_assert in configure, and set CONFIG_STATIC_ASSERT
accordingly. QEMU_BUILD_BUG_ON invokes _Static_assert if CONFIG_STATIC_ASSERT
is defined, and reverts to the old way otherwise.
That way, systems without C11 conforming compiler will still have the old
messages, as verified by intentionally breaking the configure check.
the following example output was generated by inverting the condition in
QEMU_BUILD_BUG_ON:
without _Static_assert:
> In file included from /qemu/include/qemu/osdep.h:36:0,
> from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function ‘qmp_guest_exec_status’:
> /qemu/include/qemu/compiler.h:89:12: error: negative width in bit-field ‘<anonymous>’
> struct { \
> ^
> /qemu/include/qemu/compiler.h:96:38: note: in expansion of macro QEMU_BUILD_BUG_ON_STRUCT’
> #define QEMU_BUILD_BUG_ON(x) typedef QEMU_BUILD_BUG_ON_STRUCT(x) \
> ^~~~~~~~~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro ‘QEMU_BUILD_BUG_ON’
> QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro ‘atomic_load_acquire’
> atomic_load_acquire(ptr)
> ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro ‘atomic_mb_read’
> bool finished = atomic_mb_read(&gei->finished);
> ^~~~~~~~~~~~~~
with _Static_assert:
> In file included from /qemu/include/qemu/osdep.h:36:0,
> from /qemu/qga/commands.c:13:
> /qemu/qga/commands.c: In function ‘qmp_guest_exec_status’:
> /qemu/include/qemu/compiler.h:94:30: error: static assertion failed: "not expecting: sizeof(*&gei->finished) > sizeof(void *)"
> #define QEMU_BUILD_BUG_ON(x) _Static_assert((x), #x)
> ^
> /qemu/include/qemu/atomic.h:146:5: note: in expansion of macro ‘QEMU_BUILD_BUG_ON’
> QEMU_BUILD_BUG_ON(sizeof(*ptr) > sizeof(void *)); \
> ^~~~~~~~~~~~~~~~~
> /qemu/include/qemu/atomic.h:417:5: note: in expansion of macro ‘atomic_load_acquire’
> atomic_load_acquire(ptr)
> ^~~~~~~~~~~~~~~~~~~
> /qemu/qga/commands.c:160:21: note: in expansion of macro ‘atomic_mb_read’
> bool finished = atomic_mb_read(&gei->finished);
> ^~~~~~~~~~~~~~
Backports commit 49e00a18708e27c815828d9440d5c9300d19547c from qemu
Actually enable the global memory barriers if supported by the OS.
Because only recent versions of Linux include the support, they
are disabled by default. Note that it also has to be disabled
for QEMU to run under Wine.
Before this patch, rcutorture reports 85 ns/read for my machine,
after the patch it reports 12.5 ns/read. On the other hand updates
go from 50 *micro*seconds to 20 *milli*seconds.
Backports commit a40161cbe9ccbcbab798c3e4d257c4bba99d153a from qemu
move tcg-runtime.c, translate-all.(ch) and translate-common.c into
accel/tcg/ subdirectory and updated related trace-events file.
Backports commit 244f144134d0dd182f1af8654e7f9a79fe770368 and applies
relevant changes made in db432672dc50ed86dda17ac821b7eb07411a90af and
d9bb58e51068dfc48746c6af0179926c8dc05bce from qemu
For a very long time we have used 'uname -s' as our fallback if
we don't identify the target OS using a compiler #define. This
obviously doesn't work for cross-compilation, and we've had
a comment suggesting we fix this in configure for a long time.
Since we now have an exhaustive list of which OSes we can run
on (thanks to commit 898be3e0415 making an unrecognized OS
be a fatal error), we know which ones we're missing.
Add check_define tests for the remaining OSes we support. The
defines checked are based on ones we already use in the codebase for
identifying the host OS (with the exception of GNU/kFreeBSD).
We can now set bogus_os immediately rather than doing it later.
We leave the comment about uname being bad untouched, since
there is still a use of it for the fallback for unrecognized
host CPU type.
Backports commit 951fedfceeda1b09ac8aa1f5263288b65e13caca from qemu
Currently if the user's compiler works for creating .o files but
their linker is broken such that compiling an executable from a
C file does not work, we will report a misleading error message
about the compiler not supporting __thread (since that happens
to be the first test we run which requires a working linker).
Explicitly check that compile_prog works as well as compile_object,
so that people whose toolchain setup is broken get a more helpful
error message.
Backports commit 0ef74c7496fd3c526b2259f86326eca4b3a03b78 from qemu
The change in commit 898be3e0415c6d which made completely
unrecognized OSes cause an error_exit "Unsupported host OS"
has some unfortunate unintended effects:
* if you run 'configure --help' on an unsupported host OS
(eg if intending to use it as a build machine for a
cross compile to a supported host) then the message
is printed instead of --help
* if the C compiler doesn't work or is missing (eg if
you passed an incorrect --cross-prefix by mistake)
the message is printed instead of the more useful
'compiler does not exist or does not work' message
Fix this by postponing the error_exit in this situation
until later, when we have already identified the more
useful cases for this.
The long term fix for this would be to move handling
of --help much further up in the configure script,
and make its output not dependent on checks that configure
runs. However for 2.9 this would be too invasive.
Backports commit fb59dabd4fa7e6586824ac3012073b943fc8dc79 from qemu
We plan to drop support in a future QEMU release for host OSes
and host architectures for which we have no test machine where
we can build and run tests. For the 2.9 release, make configure
print a warning if it is run on such a host, so that the user
has some warning of the plans and can volunteer to help us
maintain the port if they need it to continue to function.
This commit flags up as deprecated the CPU architectures:
* ia64
* sparc
* anything which we don't have a TCG port for
(and which was presumably using TCI)
and the OSes:
* GNU/kFreeBSD
* DragonFly BSD
* NetBSD
* OpenBSD
* Solaris
* AIX
* Haiku
It also makes entirely unrecognized host OS strings be
rejected rather than treated as if they were Linux (which
likely never worked).
Backports commit 898be3e0415c6d614395c087ef1e91210797cda7 from qemu
Solaris 9 was released in 2002, its successor Solaris 10 was
released in 2005, and Solaris 9 was end-of-lifed in 2014.
Nobody has stepped forward to express interest in supporting
Solaris of any flavour, so removing support for the ancient
versions seems uncontroversial.
In particular, this allows us to remove a use of 'uname'
in configure that won't work if you're cross-compiling.
Backports commit 91939262ffcd3c85ea6a4793d3029326eea1d649 from qemu
Clang 3.9 passes the CONFIG_AVX2_OPT configure test. However, the
supplied <cpuid.h> does not contain the bit_AVX2 define that we use
when detecting whether the routine can be enabled.
Introduce a qemu-specific header that uses the compiler's definition
of __cpuid et al, but supplies any missing bit_* definitions needed.
This avoids introducing any extra ifdefs to util/bufferiszero.c, and
allows quite a few to be removed from tcg/i386/tcg-target.inc.c.
Backports commit 5dd8990841a9e331d9d4838a116291698208cbb6 from qemu
We dropped support for ia64 host CPUs in the 2.11 release (removing
the TCG backend for it, and advertising the support as being
completely removed in the changelog). However there are a few bits
and pieces of code still floating about. Remove those, too.
We can drop the check in configure for "ia64 or hppa host?"
entirely, because we don't support hppa hosts either any more.
Backports commit b1cef6d02f84bd842fb94a6109ad4e2ad873e8e5 from qemu
Since 218bb57dd79d6843e0592c30a82ea8c1fddc74a5, the -fsanitize=address
check fails with:
config-temp/qemu-conf.c:3:20: error: integer overflow in expression [-Werror=overflow]
return INT32_MIN / -1;
Interestingly, UBSAN check doesn't produce a compile time warning.
Use a test that doesn't have compile time warnings, and make it
specific to UBSAN check.
Backports commit b9f44da2f2cdc1a1a1be5aed0c46bd7fcc69cf4a from qemu
Check for the presence of posix_memalign() in the configure script,
not using "defined(_POSIX_C_SOURCE) && !defined(__sun__)". This
lets qemu use posix_memalign() on NetBSD versions that have it,
instead of falling back to valloc() which is wasteful when the
required alignment is smaller than a page.
Backports commit 9bc5a7193fb422ee53187601eba577ee5d195522 from qemu
Typical slowdown introduced by AddressSanitizer is 2x.
UBSan shouldn't have much impact on runtime cost.
Enable it by default when --enable-debug, unless --disable-sanitizers.
Backports commit 247724cb302af5d70c8853154b640dfabf2bbb56 from qemu
GCC 4.9 and newer stopped warning for missing braces around the
"universal" C zero initializer {0}. One such initializer sneaked
into scsi/qemu-pr-helper.c and is breaking the build with such
older GCC versions.
Detect the lack of support for the idiom, and disable the warning
in that case.
Backports commit 20bc94a2b8449b7700b6bfa25a87ce2320a1c649 from qemu
Nobody has mentioned AIX host support on the mailing list for years,
and we have no test systems for it so it is most likely broken.
We've advertised in configure for two releases now that we plan
to drop support for this host OS, and have had no complaints.
Drop the AIX host support code.
We can also drop the now-unused AIX version of sys_cache_info().
Note that the _CALL_AIX define used in the PPC tcg backend is
also used for Linux PPC64, and so that code should not be removed.
Backports commit 7872375219c03682bda3f6191fa5f6a58238ed36 from qemu
This warning is included in -Wall by clang, but not by GCC (which only
enables it for -Wextra). Include it in the list of warnings we enable
to minimize the differences between the compilers:
Backports commit b98fcfd8840f290c406c32301340e96f00238a93 from qemu
This lets you build without TCG (hardware accelerationor qtest only). When
this flag is passed to configure, it will automatically filter out the target
list to only those that support KVM or Xen or HAX.
Backports commit b3f6ea7e55e8228d6f84d5cee7cb11cae917ba95 from qemu
The Cygwin target is really compiling for native Win32 with -mno-cygwin.
Except, GCC 4.7.0 has finally removed the long deprecated -mno-cygwin
option, and that happened about five years ago.
Let it rest in peace.
Backports commit c8645752ce31cc044ecc5f969a986fdcb6aab590 from qemu
This enables the multi-threaded system emulation by default for ARMv7
and ARMv8 guests using the x86_64 TCG backend. This is because on the
guest side:
- The ARM translate.c/translate-64.c have been converted to
- use MTTCG safe atomic primitives
- emit the appropriate barrier ops
- The ARM machine has been updated to
- hold the BQL when modifying shared cross-vCPU state
- defer powerctl changes to async safe work
All the host backends support the barrier and atomic primitives but
need to provide same-or-better support for normal load/store
operations.
Backports commit ca759f9e387db87e1719911f019bc60c74be9ed8 from qemu
Allow qemu to build on 32-bit hosts without 64-bit atomic ops.
Even if we only allow 32-bit hosts to multi-thread emulate 32-bit
guests, we still need some way to handle the 32-bit guest using a
64-bit atomic operation. Do so by dropping back to single-step.
Backports commit df79b996a7b21c6ea7847f7927a2e1a294b86c72 from qemu
Force the use of cmpxchg16b on x86_64.
Wikipedia suggests that only very old AMD64 (circa 2004) did not have
this instruction. Further, it's required by Windows 8 so no new cpus
will ever omit it.
If we truely care about these, then we could check this at startup time
and then avoid executing paths that use it.
Backports commit 7ebee43ee3e2fcd7b5063058b7ef74bc43216733 from qemu
