target/i386: introduce flags writeback mechanism

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>

2 files changed, 38 insertions, 1 deletions

diff --git a/tests/tcg/i386/Makefile.target b/tests/tcg/i386/Makefile.target
index 3dec7c6..9906f9e 100644
--- a/tests/tcg/i386/Makefile.target
+++ b/tests/tcg/i386/Makefile.target
@@ -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
diff --git a/tests/tcg/i386/test-flags.c b/tests/tcg/i386/test-flags.c
new file mode 100644
index 0000000..c379e29
--- /dev/null
+++ b/tests/tcg/i386/test-flags.c
@@ -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);
+}