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>

target/i386/cpu.h

@@ -1285,6 +1285,7 @@ typedef enum {
 
     CC_OP_NB,
 } CCOp;
+QEMU_BUILD_BUG_ON(CC_OP_NB >= 128);
 
 typedef struct SegmentCache {
     uint32_t selector;

target/i386/tcg/decode-new.c.inc

@@ -1662,6 +1662,7 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
     bool first = true;
     X86DecodedInsn decode;
     X86DecodeFunc decode_func = decode_root;
+    uint8_t cc_live;
 
     s->has_modrm = false;
 
@@ -1815,6 +1816,7 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
     }
 
     memset(&decode, 0, sizeof(decode));
+    decode.cc_op = -1;
     decode.b = b;
     if (!decode_insn(s, env, decode_func, &decode)) {
         goto illegal_op;
@@ -1953,6 +1955,38 @@ static void disas_insn_new(DisasContext *s, CPUState *cpu, int b)
         decode.e.gen(s, env, &decode);
         gen_writeback(s, &decode, 0, s->T0);
     }
+
+    /*
+     * Write back flags after last memory access.  Some newer ALU instructions, as
+     * well as SSE instructions, write flags in the gen_* function, but that can
+     * cause incorrect tracking of CC_OP for instructions that write to both memory
+     * and flags.
+     */
+    if (decode.cc_op != -1) {
+        if (decode.cc_dst) {
+            tcg_gen_mov_tl(cpu_cc_dst, decode.cc_dst);
+        }
+        if (decode.cc_src) {
+            tcg_gen_mov_tl(cpu_cc_src, decode.cc_src);
+        }
+        if (decode.cc_src2) {
+            tcg_gen_mov_tl(cpu_cc_src2, decode.cc_src2);
+        }
+        if (decode.cc_op == CC_OP_DYNAMIC) {
+            tcg_gen_mov_i32(cpu_cc_op, decode.cc_op_dynamic);
+        }
+        set_cc_op(s, decode.cc_op);
+        cc_live = cc_op_live[decode.cc_op];
+    } else {
+        cc_live = 0;
+    }
+    if (decode.cc_op != CC_OP_DYNAMIC) {
+        assert(!decode.cc_op_dynamic);
+        assert(!!decode.cc_dst == !!(cc_live & USES_CC_DST));
+        assert(!!decode.cc_src == !!(cc_live & USES_CC_SRC));
+        assert(!!decode.cc_src2 == !!(cc_live & USES_CC_SRC2));
+    }
+
     return;
  gp_fault:
     gen_exception_gpf(s);

target/i386/tcg/decode-new.h

@@ -283,6 +283,10 @@ struct X86DecodedInsn {
     target_ulong immediate;
     AddressParts mem;
 
+    TCGv cc_dst, cc_src, cc_src2;
+    TCGv_i32 cc_op_dynamic;
+    int8_t cc_op;
+
     uint8_t b;
 };
 

target/i386/tcg/emit.c.inc

@@ -339,6 +339,19 @@ static inline int vector_len(DisasContext *s, X86DecodedInsn *decode)
     return s->vex_l ? 32 : 16;
 }
 
+static void prepare_update1_cc(X86DecodedInsn *decode, DisasContext *s, CCOp op)
+{
+    decode->cc_dst = s->T0;
+    decode->cc_op = op;
+}
+
+static void prepare_update2_cc(X86DecodedInsn *decode, DisasContext *s, CCOp op)
+{
+    decode->cc_src = s->T1;
+    decode->cc_dst = s->T0;
+    decode->cc_op = op;
+}
+
 static void gen_store_sse(DisasContext *s, X86DecodedInsn *decode, int src_ofs)
 {
     MemOp ot = decode->op[0].ot;
@@ -1027,6 +1040,7 @@ static void gen_##uname(DisasContext *s, CPUX86State *env, X86DecodedInsn *decod
 VSIB_AVX(VPGATHERD, vpgatherd)
 VSIB_AVX(VPGATHERQ, vpgatherq)
 
+/* ADCX/ADOX do not have memory operands and can use set_cc_op.  */
 static void gen_ADCOX(DisasContext *s, CPUX86State *env, MemOp ot, int cc_op)
 {
     int opposite_cc_op;
@@ -1089,8 +1103,7 @@ static void gen_ANDN(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     MemOp ot = decode->op[0].ot;
 
     tcg_gen_andc_tl(s->T0, s->T1, s->T0);
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_LOGICB + ot);
+    prepare_update1_cc(decode, s, CC_OP_LOGICB + ot);
 }
 
 static void gen_BEXTR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
@@ -1118,10 +1131,10 @@ static void gen_BEXTR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     tcg_gen_movcond_tl(TCG_COND_LEU, s->T1, s->A0, bound, s->T1, zero);
     tcg_gen_andc_tl(s->T0, s->T0, s->T1);
 
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_LOGICB + ot);
+    prepare_update1_cc(decode, s, CC_OP_LOGICB + ot);
 }
 
+/* BLSI do not have memory operands and can use set_cc_op.  */
 static void gen_BLSI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
 {
     MemOp ot = decode->op[0].ot;
@@ -1133,6 +1146,7 @@ static void gen_BLSI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
     set_cc_op(s, CC_OP_BMILGB + ot);
 }
 
+/* BLSMSK do not have memory operands and can use set_cc_op.  */
 static void gen_BLSMSK(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
 {
     MemOp ot = decode->op[0].ot;
@@ -1144,6 +1158,7 @@ static void gen_BLSMSK(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode
     set_cc_op(s, CC_OP_BMILGB + ot);
 }
 
+/* BLSR do not have memory operands and can use set_cc_op.  */
 static void gen_BLSR(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
 {
     MemOp ot = decode->op[0].ot;
@@ -1164,18 +1179,15 @@ static void gen_BZHI(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)
 
     tcg_gen_ext8u_tl(s->T1, s->T1);
 
+    tcg_gen_shl_tl(s->A0, mone, s->T1);
+    tcg_gen_movcond_tl(TCG_COND_LEU, s->A0, s->T1, bound, s->A0, zero);
+    tcg_gen_andc_tl(s->T0, s->T0, s->A0);
     /*
      * Note that since we're using BMILG (in order to get O
      * cleared) we need to store the inverse into C.
      */
-    tcg_gen_setcond_tl(TCG_COND_LEU, cpu_cc_src, s->T1, bound);
-
-    tcg_gen_shl_tl(s->A0, mone, s->T1);
-    tcg_gen_movcond_tl(TCG_COND_LEU, s->A0, s->T1, bound, s->A0, zero);
-    tcg_gen_andc_tl(s->T0, s->T0, s->A0);
-
-    gen_op_update1_cc(s);
-    set_cc_op(s, CC_OP_BMILGB + ot);
+    tcg_gen_setcond_tl(TCG_COND_LEU, s->T1, s->T1, bound);
+    prepare_update2_cc(decode, s, CC_OP_BMILGB + ot);
 }
 
 static void gen_CRC32(DisasContext *s, CPUX86State *env, X86DecodedInsn *decode)

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

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);
+}