Re: [PATCH v3 05/12] KVM: x86/mmu: allow APICv memslot to be partially enabled

[Paolo Bonzini <pbonzini@redhat.com>]

On 02/08/21 20:33, Maxim Levitsky wrote:
> on AMD, APIC virtualization needs to dynamicaly inhibit the AVIC in a
> response to some events, and this is problematic and not efficient to do by
> enabling/disabling the memslot that covers APIC's mmio range.
> Plus due to SRCU locking, it makes it more complex to request AVIC inhibition.
> 
> Instead, the APIC memslot will be always enabled, but the MMU code
> will not install a SPTE for it, when arch.apic_access_memslot_enabled == false
> and instead jump straight to emulating the access.
> 
> When inhibiting the AVIC, this SPTE will be zapped.
> 
> This code is based on a suggestion from Sean Christopherson:
> https://lkml.org/lkml/2021/7/19/2970
> 
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 23 ++++++++++++++++++-----
>   1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6f77f6efd43c..965b562da893 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3857,11 +3857,24 @@ static bool kvm_faultin_pfn(struct kvm_vcpu *vcpu, bool prefault, gfn_t gfn,
>   	if (slot && (slot->flags & KVM_MEMSLOT_INVALID))
>   		goto out_retry;
>   
> -	/* Don't expose private memslots to L2. */
> -	if (is_guest_mode(vcpu) && !kvm_is_visible_memslot(slot)) {
> -		*pfn = KVM_PFN_NOSLOT;
> -		*writable = false;
> -		return false;
> +	if (!kvm_is_visible_memslot(slot)) {
> +		/* Don't expose private memslots to L2. */
> +		if (is_guest_mode(vcpu)) {
> +			*pfn = KVM_PFN_NOSLOT;
> +			*writable = false;
> +			return false;
> +		}
> +		/*
> +		 * If the APIC access page exists but is disabled, go directly
> +		 * to emulation without caching the MMIO access or creating a
> +		 * MMIO SPTE.  That way the cache doesn't need to be purged
> +		 * when the AVIC is re-enabled.
> +		 */
> +		if (slot && slot->id == APIC_ACCESS_PAGE_PRIVATE_MEMSLOT &&
> +		    !vcpu->kvm->arch.apic_access_memslot_enabled) {

In addition to using apicv_inhibit_reasons, I would change the subject 
to "allow APICv memslot to be enabled but invisible".  Otherwise looks good.

Paolo

> +			*r = RET_PF_EMULATE;
> +			return true;
> +		}
>   	}
>   
>   	async = false;
>