Re: [PULL 13/42] target/i386: use gen_writeback() within gen_POP()

[Paolo Bonzini]

On 7/10/24 11:42, Clément Chigot wrote:
> Hi Mark,
>
> This patch introduces regressions in our x86_64 VxWorks kernels
> running over qemu. Some page faults are triggered randomly.
>
> Earlier to this patch, the MemOp `ot` passed to `gen_op_st_v` was the
> `gen_pop_T0` created a few lines above.
> Now, this is `op->ot` which comes from elsewhere.
>
> Adding `op->ot = ot` just before calling `gen_writeback` fixes my
> regressions. But I'm wondering if there could be some unexpected
> fallbacks, `op->ot` possibly being used afterwards.

Mark's patch is correct and the (previously latent) mistake is in
the decoding table.

The manual correctly says that POP has "default 64-bit" operand;
that is, it is 64-bit even without a REX.W prefix.  It must be
marked as "d64" rather than "v".

Can you test this patch?

diff --git a/target/i386/tcg/decode-new.c.inc b/target/i386/tcg/decode-new.c.inc
index 0d846c32c22..d2da1d396d5 100644
--- a/target/i386/tcg/decode-new.c.inc
+++ b/target/i386/tcg/decode-new.c.inc
@@ -1717,7 +1717,7 @@ static const X86OpEntry opcodes_root[256] = {
     [0x8C] = X86_OP_ENTRYwr(MOV, E,v, S,w, op0_Mw),
     [0x8D] = X86_OP_ENTRYwr(LEA, G,v, M,v, nolea),
     [0x8E] = X86_OP_ENTRYwr(MOV, S,w, E,w),
-    [0x8F] = X86_OP_GROUPw(group1A, E,v),
+    [0x8F] = X86_OP_GROUPw(group1A, E,d64),
 
     [0x98] = X86_OP_ENTRY1(CBW,    0,v), /* rAX */
     [0x99] = X86_OP_ENTRYwr(CWD,   2,v, 0,v), /* rDX, rAX */
diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
index fc7477833bc..36bb308e449 100644
--- a/target/i386/tcg/emit.c.inc
+++ b/target/i386/tcg/emit.c.inc
@@ -2788,6 +2788,8 @@ static void gen_POP(DisasContext *s, X86DecodedInsn 
*decode)
     X86DecodedOp *op = &decode->op[0];
     MemOp ot = gen_pop_T0(s);
 
+    /* Only 16/32-bit access in 32-bit mode, 16/64-bit access in long mode.  */
+    assert(ot == op->ot);
     if (op->has_ea || op->unit == X86_OP_SEG) {
         /* NOTE: order is important for MMU exceptions */
         gen_writeback(s, decode, 0, s->T0);

Thanks (and it's reassuring that everything else has worked fine
for you!),

Paolo

> Thanks,
> Clément
>
> On Sat, Jun 8, 2024 at 10:36 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
> > From: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> >
> > Instead of directly implementing the writeback using gen_op_st_v(), use the
> > existing gen_writeback() function.
> >
> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> > Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> > Message-ID: <20240606095319.229650-3-mark.cave-ayland@ilande.co.uk>
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >   target/i386/tcg/emit.c.inc | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/i386/tcg/emit.c.inc b/target/i386/tcg/emit.c.inc
> > index ca78504b6e4..6123235c000 100644
> > --- a/target/i386/tcg/emit.c.inc
> > +++ b/target/i386/tcg/emit.c.inc
> > @@ -2580,9 +2580,9 @@ static void gen_POP(DisasContext *s, CPUX86State *env, 
> > X86DecodedInsn *decode)
> >
> >       if (op->has_ea) {
> >           /* NOTE: order is important for MMU exceptions */
> > -        gen_op_st_v(s, ot, s->T0, s->A0);
> > -        op->unit = X86_OP_SKIP;
> > +        gen_writeback(s, decode, 0, s->T0);
> >       }
> > +
> >       /* NOTE: writing back registers after update is important for pop %sp */
> >       gen_pop_update(s, ot);
> >   }
> > --
> > 2.45.1