[PATCH 0/2] target/i386: two fixes for cmpxchg

[PATCH 0/2] target/i386: two fixes for cmpxchg

[Paolo Bonzini]

The first was found by test-x86_64, the second by inspection.  I will
shortly work on adding golden output for the tests in tests/tcg/i386,
so that we check more than "QEMU does not crash".

Paolo


Paolo Bonzini (2):
  target/i386: fix cmpxchg with 32-bit register destination
  target/i386: cmpxchg only overwrites ZF

 target/i386/tcg/translate.c | 83 +++++++++++++++++++++++++------------
 1 file changed, 57 insertions(+), 26 deletions(-)

-- 
2.37.2

[PATCH 1/2] target/i386: fix cmpxchg with 32-bit register destination

[Paolo Bonzini]

Unlike the memory case, where "the destination operand receives a write
cycle without regard to the result of the comparison", rm must not be
touched altogether if the write fails, including not zero-extending
it on 64-bit processors.  This is not how the movcond currently works,
because it is always followed by a gen_op_mov_reg_v to rm.

To fix it, introduce a new function that is similar to gen_op_mov_reg_v
but writes to a TCG temporary.

Considering that gen_extu(ot, oldv) is not needed in the memory case
either, the two cases for register and memory destinations are different
enough that one might as well fuse the two "if (mod == 3)" into one.
So do that too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/tcg/translate.c | 73 ++++++++++++++++++++++++++-----------
 1 file changed, 51 insertions(+), 22 deletions(-)

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index d6420df31d..0e9237d627 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -429,32 +429,51 @@ static inline MemOp mo_b_d32(int b, MemOp ot)
     return b & 1 ? (ot == MO_16 ? MO_16 : MO_32) : MO_8;
 }
 
-static void gen_op_mov_reg_v(DisasContext *s, MemOp ot, int reg, TCGv t0)
+/* Compute the result of writing t0 to the OT-sized register REG.
+ *
+ * If DEST is NULL, store the result into the register and return the
+ * register's TCGv.
+ *
+ * If DEST is not NULL, store the result into DEST and return the
+ * register's TCGv.
+ */
+static TCGv gen_op_deposit_reg_v(DisasContext *s, MemOp ot, int reg, TCGv dest, TCGv t0)
 {
     switch(ot) {
     case MO_8:
-        if (!byte_reg_is_xH(s, reg)) {
-            tcg_gen_deposit_tl(cpu_regs[reg], cpu_regs[reg], t0, 0, 8);
-        } else {
-            tcg_gen_deposit_tl(cpu_regs[reg - 4], cpu_regs[reg - 4], t0, 8, 8);
+        if (byte_reg_is_xH(s, reg)) {
+            dest = dest ? dest : cpu_regs[reg - 4];
+            tcg_gen_deposit_tl(dest, cpu_regs[reg - 4], t0, 8, 8);
+            return cpu_regs[reg - 4];
         }
+        dest = dest ? dest : cpu_regs[reg];
+        tcg_gen_deposit_tl(dest, cpu_regs[reg], t0, 0, 8);
         break;
     case MO_16:
-        tcg_gen_deposit_tl(cpu_regs[reg], cpu_regs[reg], t0, 0, 16);
+        dest = dest ? dest : cpu_regs[reg];
+        tcg_gen_deposit_tl(dest, cpu_regs[reg], t0, 0, 16);
         break;
     case MO_32:
         /* For x86_64, this sets the higher half of register to zero.
            For i386, this is equivalent to a mov. */
-        tcg_gen_ext32u_tl(cpu_regs[reg], t0);
+        dest = dest ? dest : cpu_regs[reg];
+        tcg_gen_ext32u_tl(dest, t0);
         break;
 #ifdef TARGET_X86_64
     case MO_64:
-        tcg_gen_mov_tl(cpu_regs[reg], t0);
+        dest = dest ? dest : cpu_regs[reg];
+        tcg_gen_mov_tl(dest, t0);
         break;
 #endif
     default:
         tcg_abort();
     }
+    return cpu_regs[reg];
+}
+
+static void gen_op_mov_reg_v(DisasContext *s, MemOp ot, int reg, TCGv t0)
+{
+    gen_op_deposit_reg_v(s, ot, reg, NULL, t0);
 }
 
 static inline
@@ -5495,26 +5514,36 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
                                           s->mem_index, ot | MO_LE);
                 gen_op_mov_reg_v(s, ot, R_EAX, oldv);
             } else {
+                gen_extu(ot, cmpv);
                 if (mod == 3) {
+                    TCGv dest;
                     rm = (modrm & 7) | REX_B(s);
                     gen_op_mov_v_reg(s, ot, oldv, rm);
+                    gen_extu(ot, oldv);
+
+                    /*
+                     * Unlike the memory case, where "the destination operand receives
+                     * a write cycle without regard to the result of the comparison",
+                     * rm must not be touched altogether if the write fails, including
+                     * not zero-extending it on 64-bit processors.  So, precompute
+                     * the result of a successful writeback and perform the movcond
+                     * directly on cpu_regs.  Also need to write accumulator first, in
+                     * case rm is part of RAX too.
+                     */
+                    gen_op_mov_reg_v(s, ot, R_EAX, oldv);
+                    dest = gen_op_deposit_reg_v(s, ot, rm, newv, newv);
+                    tcg_gen_movcond_tl(TCG_COND_EQ, dest, oldv, cmpv, newv, dest);
                 } else {
                     gen_lea_modrm(env, s, modrm);
                     gen_op_ld_v(s, ot, oldv, s->A0);
-                    rm = 0; /* avoid warning */
-                }
-                gen_extu(ot, oldv);
-                gen_extu(ot, cmpv);
-                /* store value = (old == cmp ? new : old);  */
-                tcg_gen_movcond_tl(TCG_COND_EQ, newv, oldv, cmpv, newv, oldv);
-                if (mod == 3) {
-                    gen_op_mov_reg_v(s, ot, R_EAX, oldv);
-                    gen_op_mov_reg_v(s, ot, rm, newv);
-                } else {
-                    /* Perform an unconditional store cycle like physical cpu;
-                       must be before changing accumulator to ensure
-                       idempotency if the store faults and the instruction
-                       is restarted */
+
+                    /*
+                     * Perform an unconditional store cycle like physical cpu;
+                     * must be before changing accumulator to ensure
+                     * idempotency if the store faults and the instruction
+                     * is restarted
+                     */
+                    tcg_gen_movcond_tl(TCG_COND_EQ, newv, oldv, cmpv, newv, oldv);
                     gen_op_st_v(s, ot, newv, s->A0);
                     gen_op_mov_reg_v(s, ot, R_EAX, oldv);
                 }
-- 
2.37.2

[PATCH 2/2] target/i386: cmpxchg only overwrites ZF

[Paolo Bonzini]

Do not set all the flags, they are untouched other than ZF.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/tcg/translate.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index 0e9237d627..488a617d14 100644
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -5548,10 +5548,12 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
                     gen_op_mov_reg_v(s, ot, R_EAX, oldv);
                 }
             }
-            tcg_gen_mov_tl(cpu_cc_src, oldv);
-            tcg_gen_mov_tl(s->cc_srcT, cmpv);
-            tcg_gen_sub_tl(cpu_cc_dst, cmpv, oldv);
-            set_cc_op(s, CC_OP_SUBB + ot);
+
+            gen_compute_eflags(s);
+            tcg_gen_andi_tl(cpu_cc_src, cpu_cc_src, ~CC_Z);
+            tcg_gen_addi_tl(s->T0, cpu_cc_src, CC_Z);
+            tcg_gen_movcond_tl(TCG_COND_EQ, cpu_cc_src, oldv, cmpv, s->T0, cpu_cc_src);
+
             tcg_temp_free(oldv);
             tcg_temp_free(newv);
             tcg_temp_free(cmpv);
-- 
2.37.2

Re: [PATCH 1/2] target/i386: fix cmpxchg with 32-bit register destination

[Richard Henderson]

On 9/11/22 23:26, Paolo Bonzini wrote:
> +/* Compute the result of writing t0 to the OT-sized register REG.
> + *
> + * If DEST is NULL, store the result into the register and return the
> + * register's TCGv.
> + *
> + * If DEST is not NULL, store the result into DEST and return the
> + * register's TCGv.
> + */

/*
  * Compute...

Why bother passing NULL, and fixing it up at each use...

> +static void gen_op_mov_reg_v(DisasContext *s, MemOp ot, int reg, TCGv t0)
> +{
> +    gen_op_deposit_reg_v(s, ot, reg, NULL, t0);
>   }

... when you can just as easily pass in the register here?

>   
>   static inline
> @@ -5495,26 +5514,36 @@ static target_ulong disas_insn(DisasContext *s, CPUState *cpu)
>                                             s->mem_index, ot | MO_LE);
>                   gen_op_mov_reg_v(s, ot, R_EAX, oldv);
>               } else {
> +                gen_extu(ot, cmpv);
>                   if (mod == 3) {
> +                    TCGv dest;
>                       rm = (modrm & 7) | REX_B(s);
>                       gen_op_mov_v_reg(s, ot, oldv, rm);
> +                    gen_extu(ot, oldv);
> +
> +                    /*
> +                     * Unlike the memory case, where "the destination operand receives
> +                     * a write cycle without regard to the result of the comparison",
> +                     * rm must not be touched altogether if the write fails, including
> +                     * not zero-extending it on 64-bit processors.  So, precompute
> +                     * the result of a successful writeback and perform the movcond
> +                     * directly on cpu_regs.  Also need to write accumulator first, in
> +                     * case rm is part of RAX too.
> +                     */
> +                    gen_op_mov_reg_v(s, ot, R_EAX, oldv);
> +                    dest = gen_op_deposit_reg_v(s, ot, rm, newv, newv);
> +                    tcg_gen_movcond_tl(TCG_COND_EQ, dest, oldv, cmpv, newv, dest);
>                   } else {
>                       gen_lea_modrm(env, s, modrm);
>                       gen_op_ld_v(s, ot, oldv, s->A0);
> -                    rm = 0; /* avoid warning */
> -                }
> -                gen_extu(ot, oldv);
> -                gen_extu(ot, cmpv);
> -                /* store value = (old == cmp ? new : old);  */
> -                tcg_gen_movcond_tl(TCG_COND_EQ, newv, oldv, cmpv, newv, oldv);
> -                if (mod == 3) {
> -                    gen_op_mov_reg_v(s, ot, R_EAX, oldv);
> -                    gen_op_mov_reg_v(s, ot, rm, newv);
> -                } else {
> -                    /* Perform an unconditional store cycle like physical cpu;
> -                       must be before changing accumulator to ensure
> -                       idempotency if the store faults and the instruction
> -                       is restarted */
> +
> +                    /*
> +                     * Perform an unconditional store cycle like physical cpu;
> +                     * must be before changing accumulator to ensure
> +                     * idempotency if the store faults and the instruction
> +                     * is restarted
> +                     */
> +                    tcg_gen_movcond_tl(TCG_COND_EQ, newv, oldv, cmpv, newv, oldv);
>                       gen_op_st_v(s, ot, newv, s->A0);
>                       gen_op_mov_reg_v(s, ot, R_EAX, oldv);
>                   }

We have an outstanding bug report that suggests that the move to eax must use the deposit 
in both cases:

https://gitlab.com/qemu-project/qemu/-/issues/508


r~

Re: [PATCH 2/2] target/i386: cmpxchg only overwrites ZF

[Richard Henderson]

On 9/11/22 23:26, Paolo Bonzini wrote:
> Do not set all the flags, they are untouched other than ZF.

Um, no, second sentence:

# The ZF flag is set if the values in the destination operand and register
# AL, AX, or EAX are equal; otherwise it is
  cleared. The CF, PF, AF, SF,
# and OF flags are set according to the results of the comparison operation.


r~

Re: [PATCH 1/2] target/i386: fix cmpxchg with 32-bit register destination

[Paolo Bonzini]

On 9/12/22 09:55, Richard Henderson wrote:
> 
>   * Compute...
> 
> Why bother passing NULL, and fixing it up at each use...
> 
>> +static void gen_op_mov_reg_v(DisasContext *s, MemOp ot, int reg, TCGv 
>> t0)
>> +{
>> +    gen_op_deposit_reg_v(s, ot, reg, NULL, t0);
>>   }
> 
> ... when you can just as easily pass in the register here?

Because dest can be fixed up to either cpu_regs[reg - 4] for high-byte 
registers, or cpu_regs[reg] for everything else.

> We have an outstanding bug report that suggests that the move to eax must use the deposit in both cases:
> 
> https://gitlab.com/qemu-project/qemu/-/issues/508

Ok, so that's two bugs.  But both of them can indeed be fixed with 
gen_op_deposit_reg_v.

Paolo