linux-kernel - Re: [PATCH v2 3/3] KVM: arm64: Use generic KVM xfer to guest work function

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Date:   Fri, 30 Jul 2021 10:52:17 -0700
From:   Oliver Upton <oupton@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <Alexandru.Elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Peter Shier <pshier@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Guangyu Shi <guangyus@...gle.com>
Subject: Re: [PATCH v2 3/3] KVM: arm64: Use generic KVM xfer to guest work function

On Fri, Jul 30, 2021 at 9:56 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Jul 30, 2021, Oliver Upton wrote:
> >
> > On Fri, Jul 30, 2021 at 2:41 AM Marc Zyngier <maz@...nel.org> wrote:
> > >
> > > On Thu, 29 Jul 2021 23:09:16 +0100, Oliver Upton <oupton@...gle.com> wrote:
> > > > @@ -714,6 +715,13 @@ static bool vcpu_mode_is_bad_32bit(struct kvm_vcpu *vcpu)
> > > >               static_branch_unlikely(&arm64_mismatched_32bit_el0);
> > > >  }
> > > >
> > > > +static bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
> > > > +{
> > > > +     return kvm_request_pending(vcpu) ||
> > > > +                     need_new_vmid_gen(&vcpu->arch.hw_mmu->vmid) ||
> > > > +                     xfer_to_guest_mode_work_pending();
> > >
> > > Here's what xfer_to_guest_mode_work_pending() says:
> > >
> > > <quote>
> > >  * Has to be invoked with interrupts disabled before the transition to
> > >  * guest mode.
> > > </quote>
> > >
> > > At the point where you call this, we already are in guest mode, at
> > > least in the KVM sense.
> >
> > I believe the comment is suggestive of guest mode in the hardware
> > sense, not KVM's vcpu->mode designation. I got this from
> > arch/x86/kvm/x86.c:vcpu_enter_guest() to infer the author's
> > intentions.
>
> Yeah, the comment is referring to hardware guest mode.  The intent is to verify
> there is no work to be done before making the expensive world switch.  There's
> no meaningful interaction with vcpu->mode, on x86 it's simply more convenient
> from a code perspective to throw it into kvm_vcpu_exit_request().

Yep, the same is true for ARM as well, doing it the way it appears in
this patch allows for the recycling of the block to enable irqs and
preemption.

--
Thanks,
Oliver