[PATCH] KVM: PPC: Book3S HV: Always flush TLB in kvmppc_alloc_reset_hpt()

[PATCH] KVM: PPC: Book3S HV: Always flush TLB in kvmppc_alloc_reset_hpt()

[David Gibson]

The KVM_PPC_ALLOCATE_HTAB ioctl(), implemented by kvmppc_alloc_reset_hpt()
is supposed to completely clear and reset a guest's Hashed Page Table (HPT)
allocating or re-allocating it if necessary.

In the case where an HPT of the right size already exists and it just
zeroes it, it forces a TLB flush on all guest CPUs, to remove any stale TLB
entries loaded from the old HPT.

However, that situation can arise when the HPT is resizing as well - or
even when switching from an RPT to HPT - so those cases need a TLB flush as
well.

So, move the TLB flush to trigger in all cases except for errors.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

Paul, this is based on Paolo's KVM tree, but it should apply without
modification to pretty much any vaguely current tree.  It's a pretty
nasty bug - the case we've found hitting it in the wild is a bit
esoteric, but it could in theory affect other situations as well.

Please apply ASAP, and should probably be queued for the stable
branches as well.

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 966097232d21..51a275cc8a4d 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -159,8 +159,6 @@ long kvmppc_alloc_reset_hpt(struct kvm *kvm, int order)
 		 * Reset all the reverse-mapping chains for all memslots
 		 */
 		kvmppc_rmap_reset(kvm);
-		/* Ensure that each vcpu will flush its TLB on next entry. */
-		cpumask_setall(&kvm->arch.need_tlb_flush);
 		err = 0;
 		goto out;
 	}
@@ -176,6 +174,10 @@ long kvmppc_alloc_reset_hpt(struct kvm *kvm, int order)
 	kvmppc_set_hpt(kvm, &info);
 
 out:
+	if (err == 0)
+		/* Ensure that each vcpu will flush its TLB on next entry. */
+		cpumask_setall(&kvm->arch.need_tlb_flush);
+
 	mutex_unlock(&kvm->lock);
 	return err;
 }
-- 
2.14.3

Re: [PATCH] KVM: PPC: Book3S HV: Always flush TLB in kvmppc_alloc_reset_hpt()

[Paul Mackerras]

On Wed, Jan 10, 2018 at 05:04:39PM +1100, David Gibson wrote:
> The KVM_PPC_ALLOCATE_HTAB ioctl(), implemented by kvmppc_alloc_reset_hpt()
> is supposed to completely clear and reset a guest's Hashed Page Table (HPT)
> allocating or re-allocating it if necessary.
> 
> In the case where an HPT of the right size already exists and it just
> zeroes it, it forces a TLB flush on all guest CPUs, to remove any stale TLB
> entries loaded from the old HPT.
> 
> However, that situation can arise when the HPT is resizing as well - or
> even when switching from an RPT to HPT - so those cases need a TLB flush as
> well.
> 
> So, move the TLB flush to trigger in all cases except for errors.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  arch/powerpc/kvm/book3s_64_mmu_hv.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> Paul, this is based on Paolo's KVM tree, but it should apply without
> modification to pretty much any vaguely current tree.  It's a pretty
> nasty bug - the case we've found hitting it in the wild is a bit
> esoteric, but it could in theory affect other situations as well.
> 
> Please apply ASAP, and should probably be queued for the stable
> branches as well.

Thanks, applied to my kvm-ppc-fixes branch, and I added
cc: stable@vger.kernel.org.

Paul.