On Mon, 2014-07-28 at 15:58 +0800, Liu ping fan wrote:
> 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");
If that was the case we would have the same race in kvmppc_do_h_enter().
I think the fact that the HPTE is locked will prevent the race, ie,
HPTE_V_HVLOCK is set until hptep[0] is written to.
If I look at at the unmap case, my understanding is that it uses
kvm_unmap_rmapp() which will also lock the HPTE (try_lock_hpte)
and so shouldn't have a race vs the above code.
Or do you see a race I don't ?
Cheers,
Ben.
> Thx.
> Fan
>
> On Mon, Jul 28, 2014 at 2:42 PM, Benjamin Herrenschmidt
> <benh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, 2014-07-28 at 14:09 +0800, Liu Ping Fan wrote:
> >> 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.
> >
> >> Signed-off-by: Liu Ping Fan <pingfank@xxxxxxxxxxxxxxxxxx>
> >> ---
> >> 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);
> >
> >
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