linux-kernel - Re: [PATCH v4 1/7] KVM: VMX: Introduce generic fastpath handler

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Date:   Sat, 2 May 2020 06:52:22 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v4 1/7] KVM: VMX: Introduce generic fastpath handler

On Fri, 1 May 2020 at 22:12, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Thu, Apr 30, 2020 at 03:28:46PM +0200, Vitaly Kuznetsov wrote:
> > Wanpeng Li <kernellwp@...il.com> writes:
> >
> > > From: Wanpeng Li <wanpengli@...cent.com>
> > >
> > > Introduce generic fastpath handler to handle MSR fastpath, VMX-preemption
> > > timer fastpath etc, move it after vmx_complete_interrupts() in order that
> > > later patch can catch the case vmexit occurred while another event was
> > > being delivered to guest. There is no obversed performance difference for
> > > IPI fastpath testing after this move.
> > >
> > > Tested-by: Haiwei Li <lihaiwei@...cent.com>
> > > Cc: Haiwei Li <lihaiwei@...cent.com>
> > > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 21 ++++++++++++++++-----
> > >  1 file changed, 16 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 3ab6ca6..9b5adb4 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -6583,6 +6583,20 @@ void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)
> > >     }
> > >  }
> > >
> > > +static enum exit_fastpath_completion vmx_exit_handlers_fastpath(struct kvm_vcpu *vcpu)
> > > +{
> > > +   if (!is_guest_mode(vcpu)) {
> >
> > Nitpick: do we actually expect to have any fastpath handlers anytime
> > soon? If not, we could've written this as
> >
> >       if (is_guest_mode(vcpu))
> >               return EXIT_FASTPATH_NONE;
> >
> > and save on identation)
>
> Agreed.  An alternative approach would be to do the check in the caller, e.g.
>
>         if (is_guest_mode(vcpu))
>                 return EXIT_FASTPATH_NONE;
>
>         return vmx_exit_handlers_fastpath(vcpu);
>
> I don't have a strong preference either way.
>
> > > +           switch (to_vmx(vcpu)->exit_reason) {
> > > +           case EXIT_REASON_MSR_WRITE:
> > > +                   return handle_fastpath_set_msr_irqoff(vcpu);
> > > +           default:
> > > +                   return EXIT_FASTPATH_NONE;
> > > +           }
> > > +   }
> > > +
> > > +   return EXIT_FASTPATH_NONE;
> > > +}
> > > +
> > >  bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
> > >
> > >  static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > > @@ -6757,17 +6771,14 @@ static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > >     if (unlikely(vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> > >             return EXIT_FASTPATH_NONE;
> > >
> > > -   if (!is_guest_mode(vcpu) && vmx->exit_reason == EXIT_REASON_MSR_WRITE)
> > > -           exit_fastpath = handle_fastpath_set_msr_irqoff(vcpu);
> > > -   else
> > > -           exit_fastpath = EXIT_FASTPATH_NONE;
> > > -
> > >     vmx->loaded_vmcs->launched = 1;
> > >     vmx->idt_vectoring_info = vmcs_read32(IDT_VECTORING_INFO_FIELD);
> > >
> > >     vmx_recover_nmi_blocking(vmx);
> > >     vmx_complete_interrupts(vmx);
> > >
> > > +   exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
>
> No need for capturing the result in a local variable, just return the function
> call.

As you know later patches need to handle an local variable even
through we can make 1/7 nicer, it is just overridden.

    Wanpeng