prepopulate guest pages after write-protecting

Zdenek reported a bug where a looping "dmsetup status" eventually hangs on SMP guests.

The problem is that kvm_mmu_get_page() prepopulates the shadow MMU before write protecting the guest page tables. By doing so, it leaves a window open where the guest can mark a pte as present while the host has shadow cached such pte as "notrap". Accesses to such address will fault in the guest without the host having a chance to fix the situation.

Zdenek, can you confirm this fixes the problem?

index 5c4c166..c89bf23 100644
@@ -852,9 +852,9 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
 	sp->gfn = gfn;
 	sp->role = role;
 	hlist_add_head(&sp->hash_link, bucket);
-	vcpu->arch.mmu.prefetch_page(vcpu, sp);
 	if (!metaphysical)
 		rmap_write_protect(vcpu->kvm, gfn);
+	vcpu->arch.mmu.prefetch_page(vcpu, sp);
 	return sp;
 }
 

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2008/4/4, Marcelo Tosatti <[email protected]>:

Great - this looks like it has fixed my problems with busy looping processes - lets see if it will fix other weird behaviours I'm seeing with kvm :)

Zdenek

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Applied, thanks. Impressive catch.

Any sufficiently difficult bug is indistinguishable from a feature.


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