KVM: Push dirty information unconditionally to backup bitmap

In mark_page_dirty_in_slot(), we bail out when no running vcpu exists
and a running vcpu context is strictly required by architecture. It may
cause backwards compatible issue. Currently, saving vgic/its tables is
the only known case where no running vcpu context is expected. We may
have other unknown cases where no running vcpu context exists and it's
reported by the warning message and we bail out without pushing the
dirty information to the backup bitmap. For this, the application is
going to enable the backup bitmap for the unknown cases. However, the
dirty information can't be pushed to the backup bitmap even though the
backup bitmap is enabled for those unknown cases in the application,
until the unknown cases are added to the allowed list of non-running
vcpu context with extra code changes to the host kernel.

In order to make the new application, where the backup bitmap has been
enabled, to work with the unchanged host, we continue to push the dirty
information to the backup bitmap instead of bailing out early. With the
added check on 'memslot->dirty_bitmap' to mark_page_dirty_in_slot(), the
kernel crash is avoided silently by the combined conditions: no running
vcpu context, kvm_arch_allow_write_without_running_vcpu() returns 'true',
and the backup bitmap (KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP) isn't enabled
yet.

Suggested-by: Sean Christopherson <[email protected]>
Signed-off-by: Gavin Shan <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

virt/kvm/kvm_main.c

@@ -3308,8 +3308,7 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
 	if (WARN_ON_ONCE(vcpu && vcpu->kvm != kvm))
 		return;
 
-	if (WARN_ON_ONCE(!kvm_arch_allow_write_without_running_vcpu(kvm) && !vcpu))
-		return;
+	WARN_ON_ONCE(!vcpu && !kvm_arch_allow_write_without_running_vcpu(kvm));
 #endif
 
 	if (memslot && kvm_slot_dirty_track_enabled(memslot)) {
@@ -3318,7 +3317,7 @@ void mark_page_dirty_in_slot(struct kvm *kvm,
 
 		if (kvm->dirty_ring_size && vcpu)
 			kvm_dirty_ring_push(vcpu, slot, rel_gfn);
-		else
+		else if (memslot->dirty_bitmap)
 			set_bit_le(rel_gfn, memslot->dirty_bitmap);
 	}
 }