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

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Message-ID: <YgWbgfSrzAhd97LG@google.com>
Date:   Thu, 10 Feb 2022 23:10:57 +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 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;
	}

>  		return false;
>  
>  	if (!kvm_pv_async_pf_enabled(vcpu) ||
> -- 
> 2.31.1
> 
>