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

[Liu ping fan]

On Tue, Jul 29, 2014 at 2:57 PM, Benjamin Herrenschmidt
<benh at kernel.crashing.org> wrote:
> 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.
>
Yes, you are right :)

Thx,
Fan


> 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 at kernel.crashing.org> 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 at linux.vnet.ibm.com>
>> >> ---
>> >>  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);
>> >
>> >
>
>