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Wed, 04 Mar 2020 07:18:26 -0800 (PST) Date: Wed, 4 Mar 2020 10:18:25 -0500 From: Peter Xu To: Paolo Bonzini Cc: linmiaohe , Sean Christopherson , Vitaly Kuznetsov , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" Subject: Re: [PATCH] KVM: X86: Avoid explictly fetch instruction in x86_decode_insn() Message-ID: <20200304151825.GA7146@xz-x1> References: <05ca4e7e070844dd92e4f673a1bc15d9@huawei.com> <593e16d8-1021-29ef-11d0-a72d762db057@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <593e16d8-1021-29ef-11d0-a72d762db057@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 04, 2020 at 08:30:49AM +0100, Paolo Bonzini wrote: > On 04/03/20 03:37, linmiaohe wrote: > > Hi: > > Peter Xu 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 > >> --- > >> 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