Hope I am right.  Take the following seq as an example

if (hptep[0] & HPTE_V_VALID) {
/* HPTE was previously valid, so we need to invalidate it */
unlock_rmap(rmap);
hptep[0] |= HPTE_V_ABSENT;
kvmppc_invalidate_hpte(kvm, hptep, index);
/* don't lose previous R and C bits */
r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
} else {
kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
}
  ---------------------------------------------> if we try_to_unmap on
pfn at here, then @r contains a invalid pfn
hptep[1] = r;
eieio();
hptep[0] = hpte[0];
asm volatile("ptesync" : : : "memory");

Thx.
Fan

On Mon, Jul 28, 2014 at 2:42 PM, Benjamin Herrenschmidt
[off-list ref] wrote:

On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:

quoted

In current code, the setup of hpte is under the risk of race with
mmu_notifier_invalidate, i.e we may setup a hpte with a invalid pfn.
Resolve this issue by sync the two actions by KVMPPC_RMAP_LOCK_BIT.

Please describe the race you think you see. I'm quite sure both Paul and
I went over that code and somewhat convinced ourselves that it was ok
but it's possible that we were both wrong :-)

Cheers,
Ben.

quoted

Signed-off-by: Liu Ping Fan <redacted>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index 8056107..e6dcff4 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c

@@ -754,19 +754,24 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,

      if (hptep[0] & HPTE_V_VALID) {
              /* HPTE was previously valid, so we need to invalidate it */
-             unlock_rmap(rmap);
              hptep[0] |= HPTE_V_ABSENT;
              kvmppc_invalidate_hpte(kvm, hptep, index);
              /* don't lose previous R and C bits */
              r |= hptep[1] & (HPTE_R_R | HPTE_R_C);
+
+             hptep[1] = r;
+             eieio();
+             hptep[0] = hpte[0];
+             asm volatile("ptesync" : : : "memory");
+             unlock_rmap(rmap);
      } else {
+             hptep[1] = r;
+             eieio();
+             hptep[0] = hpte[0];
+             asm volatile("ptesync" : : : "memory");
              kvmppc_add_revmap_chain(kvm, rev, rmap, index, 0);
      }

-     hptep[1] = r;
-     eieio();
-     hptep[0] = hpte[0];
-     asm volatile("ptesync" : : : "memory");
      preempt_enable();
      if (page && hpte_is_writable(r))
              SetPageDirty(page);