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

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Date:   Wed, 4 Mar 2020 02:37:06 +0000
From:   linmiaohe <linmiaohe@...wei.com>
To:     Peter Xu <peterx@...hat.com>
CC:     Paolo Bonzini <pbonzini@...hat.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()

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:

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);