linux-kernel - Re: [PATCH] KVM: VMX: restore vmx

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Message-ID: <2A2A9B8D-6941-49D3-87F8-4AAE09E6F5C3@nutanix.com>
Date:   Wed, 31 May 2023 19:44:46 +0000
From:   Jon Kohler <jon@...anix.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
CC:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "kvm @ vger . kernel . org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: restore vmx_vmexit alignment



> On May 31, 2023, at 3:42 PM, Alexey Dobriyan <adobriyan@...il.com> wrote:
> 
> On Wed, May 31, 2023 at 11:20:31AM -0700, Sean Christopherson wrote:
>> On Wed, May 31, 2023, Jon Kohler wrote:
>>> Commit 8bd200d23ec4 ("KVM: VMX: Flatten __vmx_vcpu_run()") changed
>>> vmx_vmexit from SYM_FUNC_START to SYM_INNER_LABEL, accidentally
>>> removing 16 byte alignment as SYM_FUNC_START uses SYM_A_ALIGN and
>>> SYM_INNER_LABEL does not. Josh mentioned [1] this was unintentional.
>> 
>> Anyone know if this is this stable material, or just nice to have?
> 
> Can this improve vmexit latency? I didn't measure it.

Exit latency *appeared* to be the same before/after, might be a little bit better but
any improvement appeared to be lost in the noise. I didn’t see a regression though,
so thats nice. That puts it in the nice-to-have camp for me.

Jon