Subject: Re: [PATCH] powerpc: kvm: make the setup of hpte under the protection of KVMPPC_RMAP_LOCK_BIT

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 29 Jul 2014 16:57:02 +1000

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
> <[email-masked]> 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 <[email-masked]>
> >> ---
> >>  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);
> >
> >