linux-kernel - Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

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Date:   Tue, 12 Nov 2019 09:18:11 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath

On Mon, 11 Nov 2019 at 21:06, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Wanpeng Li <kernellwp@...il.com> writes:
>
> > +
> >  static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >  {
> >       struct vcpu_vmx *vmx = to_vmx(vcpu);
> > @@ -6615,6 +6645,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> >                                 | (1 << VCPU_EXREG_CR3));
> >       vcpu->arch.regs_dirty = 0;
> >
> > +     vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> > +     vcpu->fast_vmexit = false;
> > +     if (!is_guest_mode(vcpu) &&
> > +             vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> > +             vcpu->fast_vmexit = handle_ipi_fastpath(vcpu);
>
> I have to admit this looks too much to me :-( Yes, I see the benefits of
> taking a shortcut (by actualy penalizing all other MSR writes) but the
> question I have is: where do we stop?

In our iaas environment observation, ICR and TSCDEADLINE are the main
MSR write vmexits.

Before patch:
tscdeadline_immed 3900
tscdeadline 5413

After patch:
tscdeadline_immed 3912
tscdeadline 5427

So the penalize can be tolerated.

>
> Also, this 'shortcut' creates an imbalance in tracing: you don't go down
> to kvm_emulate_wrmsr() so handle_ipi_fastpath() should probably gain a
> tracepoint.

Agreed.

>
> Looking at 'fast_vmexit' name makes me think this is something
> generic. Is this so? Maybe we can create some sort of an infrastructure
> for fast vmexit handling and make it easy to hook things up to it?

Maybe an indirect jump? But I can have a try.

    Wanpeng