Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode_insn()

[Peter Xu <peterx@redhat.com>]

On Wed, Mar 04, 2020 at 02:37:06AM +0000, linmiaohe wrote:
> Hi:
> Peter Xu <peterx@redhat.com> writes:
> >insn_fetch() will always implicitly refill instruction buffer properly when the buffer is empty, so we don't need to explicitly fetch it even if insn_len==0 for x86_decode_insn().
> >
> >Signed-off-by: Peter Xu <peterx@redhat.com>
> >---
> > arch/x86/kvm/emulate.c | 5 -----
> > 1 file changed, 5 deletions(-)
> >
> >diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c index dd19fb3539e0..04f33c1ca926 100644
> >--- a/arch/x86/kvm/emulate.c
> >+++ b/arch/x86/kvm/emulate.c
> >@@ -5175,11 +5175,6 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
> > 	ctxt->opcode_len = 1;
> > 	if (insn_len > 0)
> > 		memcpy(ctxt->fetch.data, insn, insn_len);
> >-	else {
> >-		rc = __do_insn_fetch_bytes(ctxt, 1);
> >-		if (rc != X86EMUL_CONTINUE)
> >-			goto done;
> >-	}
> > 
> > 	switch (mode) {
> > 	case X86EMUL_MODE_REAL:
> 
> Looks good, thanks. But it seems we should also take care of the comment in __do_insn_fetch_bytes(), as we do not
> load instruction at the beginning of x86_decode_insn() now, which may be misleading:
> 		/*
>          * One instruction can only straddle two pages,
>          * and one has been loaded at the beginning of
>          * x86_decode_insn.  So, if not enough bytes
>          * still, we must have hit the 15-byte boundary.
>          */
>         if (unlikely(size < op_size))
>                 return emulate_gp(ctxt, 0);

Right, thanks for spotting that (even if the patch to be dropped :).

I guess not only the comment, but the check might even fail if we
apply the patch. Because when the fetch is the 1st attempt and
unluckily that acrosses one page boundary (because we'll only fetch
until either 15 bytes or the page boundary), so that single fetch
could be smaller than op_size provided.

Thanks,

-- 
Peter Xu