Commit b8b9156e authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
Browse files

KVM: x86/mmu: Comment FNAME(sync_page) to document TLB flushing logic 



Add a comment to FNAME(sync_page) to explain why the TLB flushing logic
conspiculously doesn't handle the scenario of guest protections being
reduced.  Specifically, if synchronizing a SPTE drops execute protections,
KVM will not emit a TLB flush, whereas dropping writable or clearing A/D
bits does trigger a flush via mmu_spte_update().  Architecturally, until
the GPTE is implicitly or explicitly flushed from the guest's perspective,
KVM is not required to flush any old, stale translations.

Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Reviewed-by: default avatarJim Mattson <jmattson@google.com>
Message-Id: <20220513195000.99371-3-seanjc@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 9fb35657

arch/x86/kvm/mmu/paging_tmpl.h

+9 −0
Original line number Diff line number Diff line
@@ -1077,6 +1077,15 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp) 
		flush |= mmu_spte_update(sptep, spte);

	}



	/*

	 * Note, any flush is purely for KVM's correctness, e.g. when dropping

	 * an existing SPTE or clearing W/A/D bits to ensure an mmu_notifier

	 * unmap or dirty logging event doesn't fail to flush.  The guest is

	 * responsible for flushing the TLB to ensure any changes in protection

	 * bits are recognized, i.e. until the guest flushes or page faults on

	 * a relevant address, KVM is architecturally allowed to let vCPUs use

	 * cached translations with the old protection bits.

	 */

	return flush;

}