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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Wed, 30 Mar 2022 09:17:28 +0800
Message-ID: <CANRm+Cy66YAyRp0JJuoyp3k-D9HSZbYF3hYO3Vjxz5w1Rz-P3g@mail.gmail.com>
Subject: Re: [PATCH RESEND 2/5] KVM: X86: Add guest interrupt disable state support
To:     Sean Christopherson <seanjc@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
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On Wed, 30 Mar 2022 at 08:04, Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Mar 25, 2022, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.com>
> >
> > Let's get the information whether or not guests disable interruptions.
>
> This is missing critical information for _why_.  It took me some staring to
> understand that this allows querying IRQs from a _different_ vCPU, which needs
> caching on VMX due to the need to do a VMREAD.

Yes.

>
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h | 1 +
> >  arch/x86/kvm/x86.c              | 3 +++
> >  2 files changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 50f011a7445a..8e05cbfa9827 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -861,6 +861,7 @@ struct kvm_vcpu_arch {
> >               bool preempt_count_enabled;
> >               struct gfn_to_hva_cache preempt_count_cache;
> >       } pv_pc;
> > +     bool irq_disabled;
>
> This is going to at best be confusing, and at worst lead to bugs  The flag is
> valid if and only if the vCPU is not loaded.  I don't have a clever answer, but
> this needs to have some form of guard to (a) clarify when it's valid and (b) actively
> prevent misuse.

How about renaming it to last_guest_irq_disabled and comments as /*
Guest irq disabled state, valid iff the vCPU is not loaded */

    Wanpeng