linux-kernel - Re: [PATCH 1/5] KVM: x86: Get vmcs12 pages before checking pending interrupts

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Message-ID: <Ywa+QL/kDp9ibkbC@google.com>
Date:   Thu, 25 Aug 2022 00:11:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     Mingwei Zhang <mizhang@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH 1/5] KVM: x86: Get vmcs12 pages before checking pending
 interrupts

On Wed, Aug 03, 2022, Maxim Levitsky wrote:
> On Wed, 2022-08-03 at 10:51 -0700, Mingwei Zhang wrote:
> > On Wed, Aug 3, 2022 at 10:18 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> > > On 8/3/22 01:07, Mingwei Zhang wrote:
> > > > +     /*
> > > > +      * We must first get the vmcs12 pages before checking for interrupts
> > > > +      * that might unblock the guest if L1 is using virtual-interrupt
> > > > +      * delivery.
> > > > +      */
> > > > +     if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
> > > > +             /*
> > > > +              * If we have to ask user-space to post-copy a page,
> > > > +              * then we have to keep trying to get all of the
> > > > +              * VMCS12 pages until we succeed.
> > > > +              */
> > > > +             if (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
> > > > +                     kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
> > > > +                     return 0;
> > > > +             }
> > > > +     }
> > > > +
> > > 
> > > I think request handling (except for KVM_REQ_EVENT) could be more
> > > generically moved from vcpu_enter_guest() to vcpu_run().
> > 
> > Yeah, sounds good to me. I can come up with an updated version. At
> > least, I will remove the repeat request here.
> 
> Now it all makes sense. I do think that KVM_REQ_GET_NESTED_STATE_PAGES processing
> when the vCPU is halted is indeed missing.
> 
> This reminds me that I would be *very* happy to remove the KVM_REQ_GET_NESTED_STATE_PAGES,
> if by any chance there is an agreement to do so upstream.
> This is yet another reason to do so to be honest.
> Just my 0.2 cents of course.

+100

@google folks, what would it take for us to mark KVM_REQ_GET_NESTED_STATE_PAGES
as deprecated in upstream and stop accepting patches/fixes?  IIUC, when we eventually
move to userfaultfd, all this goes away, i.e. we do want to ditch this at some point.