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Commit 1e5bf454 authored by Paul Mackerras's avatar Paul Mackerras Committed by Alexander Graf
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KVM: PPC: Book3S HV: Fix race in reading change bit when removing HPTE 



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 6f22bd32
("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: 6f22bd32
Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
parent b4deba5c

arch/powerpc/kvm/book3s_hv_rm_mmu.c

+12 −6
Original line number Original line Diff line number Diff line
@@ -421,14 +421,20 @@ long kvmppc_do_h_remove(struct kvm *kvm, unsigned long flags, 
	rev = real_vmalloc_addr(&kvm->arch.revmap[pte_index]);

	rev = real_vmalloc_addr(&kvm->arch.revmap[pte_index]);

	v = pte & ~HPTE_V_HVLOCK;

	v = pte & ~HPTE_V_HVLOCK;

	if (v & HPTE_V_VALID) {

	if (v & HPTE_V_VALID) {

		u64 pte1;



		pte1 = be64_to_cpu(hpte[1]);

		hpte[0] &= ~cpu_to_be64(HPTE_V_VALID);

		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);

		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);

		 * 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 re-read the HPTE after

		 * the invalidation sequence has completed in order to

		 * obtain reliable values of R and C.

		 */

		remove_revmap_chain(kvm, pte_index, rev, v,

				    be64_to_cpu(hpte[1]));

	}

	}

	r = rev->guest_rpte & ~HPTE_GR_RESERVED;

	r = rev->guest_rpte & ~HPTE_GR_RESERVED;

	note_hpte_modification(kvm, rev);

	note_hpte_modification(kvm, rev);