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Subject: Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in
 x86_decode_insn()
To:     linmiaohe <linmiaohe@huawei.com>, Peter Xu <peterx@redhat.com>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
References: <05ca4e7e070844dd92e4f673a1bc15d9@huawei.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <593e16d8-1021-29ef-11d0-a72d762db057@redhat.com>
Date:   Wed, 4 Mar 2020 08:30:49 +0100
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On 04/03/20 03:37, 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:

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.

Paolo

> 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);
>