Re: [PATCH 1/3] KVM: PPC: Book3S HV: Fix race in reading change bit when removing HPTE

[Paul Mackerras <paulus@samba.org>]

On Tue, Apr 28, 2015 at 10:36:52AM +0530, Aneesh Kumar K.V wrote:
> Paul Mackerras <paulus@samba.org> writes:
> 
> > The reference (R) and change (C) bits in a HPT entry can be set by
> > hardware at any time up until the HPTE is invalidated and the TLB
> > invalidation sequence has completed.  This means that when removing
> > a HPTE, we need to read the HPTE after the invalidation sequence has
> > completed in order to obtain reliable values of R and C.  The code
> > in kvmppc_do_h_remove() used to do this.  However, commit 6f22bd3265fb
> > ("KVM: PPC: Book3S HV: Make HTAB code LE host aware") removed the
> > read after invalidation as a side effect of other changes.  This
> > restores the read of the HPTE after invalidation.
> >
> > The user-visible effect of this bug would be that when migrating a
> > guest, there is a small probability that a page modified by the guest
> > and then unmapped by the guest might not get re-transmitted and thus
> > the destination might end up with a stale copy of the page.
> >
> > Fixes: 6f22bd3265fb
> > Cc: stable@vger.kernel.org # v3.17+
> > Signed-off-by: Paul Mackerras <paulus@samba.org>
> > ---
> >  arch/powerpc/kvm/book3s_hv_rm_mmu.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv_rm_mmu.c b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > index f6bf0b1..5c1737f 100644
> > --- a/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > +++ b/arch/powerpc/kvm/book3s_hv_rm_mmu.c
> > @@ -413,14 +413,12 @@ long kvmppc_do_h_remove(struct kvm *kvm, unsigned long flags,
> >  	rev = real_vmalloc_addr(&kvm->arch.revmap[pte_index]);
> >  	v = pte & ~HPTE_V_HVLOCK;
> >  	if (v & HPTE_V_VALID) {
> > -		u64 pte1;
> > -
> > -		pte1 = be64_to_cpu(hpte[1]);
> >  		hpte[0] &= ~cpu_to_be64(HPTE_V_VALID);
> > -		rb = compute_tlbie_rb(v, pte1, pte_index);
> > +		rb = compute_tlbie_rb(v, be64_to_cpu(hpte[1]), pte_index);
> >  		do_tlbies(kvm, &rb, 1, global_invalidates(kvm, flags), true);
> >  		/* Read PTE low word after tlbie to get final R/C values */
> > -		remove_revmap_chain(kvm, pte_index, rev, v, pte1);
> > +		remove_revmap_chain(kvm, pte_index, rev, v,
> > +				    be64_to_cpu(hpte[1]));
> >  	}
> 
> May be add the above commit message as a code comment ?

Well, that's what "/* Read PTE low word after tlbie to get final R/C
values */" was trying to be, originally, but maybe it would be helpful
to expand on it.

Paul.