linux-kernel - Re: [PATCH 03/12] KVM: x86: do not deliver asynchronous page faults if CR0.PG=0

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Message-ID: <YgWcS/0naKPdAn2E@google.com>
Date:   Thu, 10 Feb 2022 23:14:19 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, mlevitsk@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 03/12] KVM: x86: do not deliver asynchronous page faults
 if CR0.PG=0

On Thu, Feb 10, 2022, Sean Christopherson wrote:
> On Wed, Feb 09, 2022, Paolo Bonzini wrote:
> > Enabling async page faults is nonsensical if paging is disabled, but
> > it is allowed because CR0.PG=0 does not clear the async page fault
> > MSR.  Just ignore them and only use the artificial halt state,
> > similar to what happens in guest mode if async #PF vmexits are disabled.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> > ---
> >  arch/x86/kvm/x86.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5e1298aef9e2..98aca0f2af12 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12272,7 +12272,9 @@ static inline bool apf_pageready_slot_free(struct kvm_vcpu *vcpu)
> >  
> >  static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
> >  {
> > -	if (!vcpu->arch.apf.delivery_as_pf_vmexit && is_guest_mode(vcpu))
> > +	if (is_guest_mode(vcpu)
> > +	    ? !vcpu->arch.apf.delivery_as_pf_vmexit
> > +	    : !is_cr0_pg(vcpu->arch.mmu))
> 
> As suggested in the previous patch, is_paging(vcpu).
> 
> I find a more tradition if-elif marginally easier to understand the implication
> that CR0.PG is L2's CR0 and thus irrelevant if is_guest_mode()==true.  Not a big
> deal though.
> 
> 	if (is_guest_mode(vcpu)) {
> 		if (!vcpu->arch.apf.delivery_as_pf_vmexit)
> 			return false;
> 	} else if (!is_paging(vcpu)) {
> 		return false;
> 	}

Alternatively, what about reordering and refactoring to yield:

	if (kvm_pv_async_pf_enabled(vcpu))
		return false;

	if (vcpu->arch.apf.send_user_only &&
	    static_call(kvm_x86_get_cpl)(vcpu) == 0)
		return false;

	/* L1 CR0.PG=1 is guaranteed if the vCPU is in guest mode (L2). */
	if (is_guest_mode(vcpu)
		return !vcpu->arch.apf.delivery_as_pf_vmexit;

	return is_cr0_pg(vcpu->arch.mmu);

There isn't any need to "batch" the if statements.