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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

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 ?

>  	r = rev->guest_rpte & ~HPTE_GR_RESERVED;
>  	note_hpte_modification(kvm, rev);
> -- 
> 2.1.4
>
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