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

From: Oliver Upton <oupton@google.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <Alexandru.Elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	Peter Shier <pshier@google.com>,
	Shakeel Butt <shakeelb@google.com>,
	Guangyu Shi <guangyus@google.com>
Subject: Re: [PATCH v2 3/3] KVM: arm64: Use generic KVM xfer to guest work function
Date: Fri, 30 Jul 2021 10:52:17 -0700	[thread overview]
Message-ID: <CAOQ_Qsjzp785QCnu9rzkuEMH9Q7tee-edJpKNgD_14t3NpaS5w@mail.gmail.com> (raw)
In-Reply-To: <YQQvT7vAnRrcAcx/@google.com>

On Fri, Jul 30, 2021 at 9:56 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Jul 30, 2021, Oliver Upton wrote:
> >
> > On Fri, Jul 30, 2021 at 2:41 AM Marc Zyngier <maz@kernel.org> wrote:
> > >
> > > On Thu, 29 Jul 2021 23:09:16 +0100, Oliver Upton <oupton@google.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