linux-kernel - Re: [PATCH RESEND 2/5] KVM: X86: Add guest interrupt disable state support

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Date:   Thu, 31 Mar 2022 22:00:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        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>
Subject: Re: [PATCH RESEND 2/5] KVM: X86: Add guest interrupt disable state
 support

On Wed, Mar 30, 2022, Wanpeng Li wrote:
> On Wed, 30 Mar 2022 at 08:04, Sean Christopherson <seanjc@...gle.com> wrote:
> > > 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 */

What about usurping vcpu->run->if_flag?  Userspace could manipulate the data, but
that should be fine since the data is already guest-controlled.