linux-kernel - Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode

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Date:   Wed, 4 Mar 2020 10:18:25 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linmiaohe <linmiaohe@...wei.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in
 x86_decode_insn()

On Wed, Mar 04, 2020 at 08:30:49AM +0100, Paolo Bonzini wrote:
> On 04/03/20 03:37, linmiaohe wrote:
> > Hi:
> > Peter Xu <peterx@...hat.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@...hat.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:
> 
> This is a a small (but measurable) optimization; going through
> __do_insn_fetch_bytes instead of do_insn_fetch_bytes is a little bit
> faster because it lets you mark the branch in do_insn_fetch_bytes as
> unlikely, and in general it allows the branch predictor to do a better job.

Ah I see, that makes sense.  Thanks!

-- 
Peter Xu