linux-kernel - Re: [PATCH v2 02/18] KVM: x86: do not deliver asynchronous page faults if CR0.PG=0

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Message-ID: <7d0787b44a484941964c2e5d4e85c1a60c73602c.camel@redhat.com>
Date:   Wed, 23 Feb 2022 16:07:18 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com
Subject: Re: [PATCH v2 02/18] KVM: x86: do not deliver asynchronous page
 faults if CR0.PG=0

On Thu, 2022-02-17 at 16:03 -0500, 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.

Well in theory someone could use KVM for emulating DOS programs, and
use async #PF for on demand paging. I would question sanity of author
of such hypervisor though...


The only thing I would add is to add a mention of the CR0.PG=1 restriction in 
Documentation/virt/kvm/msr.rst.

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

Best regards,
	Maxim Levitsky

> 
> Given the increasingly complex logic, and the nicer code if the new
> "if" is placed last, opportunistically change the "||" into a chain
> of "if (...) return false" statements.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 99a58c25f5c2..b912eef5dc1a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12270,14 +12270,28 @@ 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 (!kvm_pv_async_pf_enabled(vcpu))
>  		return false;
>  
> -	if (!kvm_pv_async_pf_enabled(vcpu) ||
> -	    (vcpu->arch.apf.send_user_only && static_call(kvm_x86_get_cpl)(vcpu) == 0))
> +	if (vcpu->arch.apf.send_user_only &&
> +	    static_call(kvm_x86_get_cpl)(vcpu) == 0)
>  		return false;
>  
> -	return true;
> +	if (is_guest_mode(vcpu)) {
> +		/*
> +		 * L1 needs to opt into the special #PF vmexits that are
> +		 * used to deliver async page faults.
> +		 */
> +		return vcpu->arch.apf.delivery_as_pf_vmexit;
> +	} else {
> +		/*
> +		 * Play it safe in case the guest does a quick real mode
> +		 * foray.  The real mode IDT is unlikely to have a #PF
> +		 * exception setup.
> +		 */
> +		return is_paging(vcpu);
> +	}
>  }
>  
>  bool kvm_can_do_async_pf(struct kvm_vcpu *vcpu)