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target/i386: introduce flags writeback mechanism

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ALU instructions can write to both memory and flags.  If the CC_SRC*
and CC_DST locations have been written already when a memory access
causes a fault, the value in CC_SRC* and CC_DST might be interpreted
with the wrong CC_OP (the one that is in effect before the instruction.

Besides just using the wrong result for the flags, something like
subtracting -1 can have disastrous effects if the current CC_OP is
CC_OP_EFLAGS: this is because QEMU does not expect bits outside the ALU
flags to be set in CC_SRC, and env->eflags can end up set to all-ones.
In the case of the attached testcase, this sets IOPL to 3 and would
cause an assertion failure if SUB is moved to the new decoder.

This mechanism is not really needed for BMI instructions, which can
only write to a register, but put it to use anyway for cleanliness.
In the case of BZHI, the code has to be modified slightly to ensure
that decode->cc_src is written, otherwise the new assertions trigger.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2023-10-11 15:26:40 +02:00
parent 4b2baf4a55
commit e7bbb7cb71
6 changed files with 101 additions and 13 deletions
Download patch file Download diff file Expand all files Collapse all files

2
tests/tcg/i386/Makefile.target
View file

@ -13,7 +13,7 @@ config-cc.mak: Makefile
I386_SRCS=$(notdir $(wildcard $(I386_SRC)/*.c))
ALL_X86_TESTS=$(I386_SRCS:.c=)
SKIP_I386_TESTS=test-i386-ssse3 test-avx test-3dnow test-mmx
SKIP_I386_TESTS=test-i386-ssse3 test-avx test-3dnow test-mmx test-flags
X86_64_TESTS:=$(filter test-i386-adcox test-i386-bmi2 $(SKIP_I386_TESTS), $(ALL_X86_TESTS))
test-i386-sse-exceptions: CFLAGS += -msse4.1 -mfpmath=sse

37
tests/tcg/i386/test-flags.c Normal file
View file

@ -0,0 +1,37 @@
#define _GNU_SOURCE
#include <sys/mman.h>
#include <signal.h>
#include <stdio.h>
#include <assert.h>
volatile unsigned long flags;
volatile unsigned long flags_after;
int *addr;
void sigsegv(int sig, siginfo_t *info, ucontext_t *uc)
{
flags = uc->uc_mcontext.gregs[REG_EFL];
mprotect(addr, 4096, PROT_READ|PROT_WRITE);
}
int main()
{
struct sigaction sa = { .sa_handler = (void *)sigsegv, .sa_flags = SA_SIGINFO };
sigaction(SIGSEGV, &sa, NULL);
/* fault in the page then protect it */
addr = mmap (NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
*addr = 0x1234;
mprotect(addr, 4096, PROT_READ);
asm("# set flags to all ones \n"
"mov $-1, %%eax \n"
"movq addr, %%rdi \n"
"sahf \n"
"sub %%eax, (%%rdi) \n"
"pushf \n"
"pop flags_after(%%rip) \n" : : : "eax", "edi", "memory");
/* OF can have any value before the SUB instruction. */
assert((flags & 0xff) == 0xd7 && (flags_after & 0x8ff) == 0x17);
}