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target-arm: Correct handling of writes to CPSR mode bits from gdb in usermode

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In helper.c the expression
(env->uncached_cpsr & CPSR_M) != CPSR_USER
is always true; the right hand side was supposed to be ARM_CPU_MODE_USR
(an error in commit cb01d391).

Since the incorrect expression was always true, this just meant that
commit cb01d391 had no effect.

However simply changing the RHS here would reveal a logic error: if
the mode is USR we wish to completely ignore the attempt to set the
mode bits, which means that we must clear the CPSR_M bits from mask
to avoid the uncached_cpsr bits being updated at the end of the
function.

Move the condition into the correct place in the code, fix its RHS
constant, and add a comment about the fact that we must be doing a
gdbstub write if we're in user mode.

Backports commit 8c4f0eb94cc65ee32a12feba88d0b32e3665d5ea from qemu
This commit is contained in:
Peter Maydell 2018-02-21 01:57:23 -05:00 committed by Lioncash
parent d111e2df2d
commit 64a9bec68a
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GPG key ID: 4E3C3CC1031BA9C7
1 changed files with 9 additions and 2 deletions
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11
qemu/target-arm/helper.c
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@ -4736,9 +4736,16 @@ void cpsr_write(CPUARMState *env, uint32_t val, uint32_t mask,
env->daif |= val & CPSR_AIF & mask; env->daif |= val & CPSR_AIF & mask;
if (write_type != CPSRWriteRaw && if (write_type != CPSRWriteRaw &&
(env->uncached_cpsr & CPSR_M) != CPSR_USER &&
((env->uncached_cpsr ^ val) & mask & CPSR_M)) { ((env->uncached_cpsr ^ val) & mask & CPSR_M)) {
if (bad_mode_switch(env, val & CPSR_M, write_type)) { if ((env->uncached_cpsr & CPSR_M) == ARM_CPU_MODE_USR) {
/* Note that we can only get here in USR mode if this is a
* gdb stub write; for this case we follow the architectural
* behaviour for guest writes in USR mode of ignoring an attempt
* to switch mode. (Those are caught by translate.c for writes
* triggered by guest instructions.)
*/
mask &= ~CPSR_M;
} else if (bad_mode_switch(env, val & CPSR_M, write_type)) {
/* Attempt to switch to an invalid mode: this is UNPREDICTABLE in /* Attempt to switch to an invalid mode: this is UNPREDICTABLE in
* v7, and has defined behaviour in v8: * v7, and has defined behaviour in v8:
* + leave CPSR.M untouched * + leave CPSR.M untouched