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    Subject[PATCH 3.16 152/306] arm64: KVM: Take S1 walks into account when determining S2 write faults
    3.16.40-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Will Deacon <will.deacon@arm.com>

    commit 60e21a0ef54cd836b9eb22c7cb396989b5b11648 upstream.

    The WnR bit in the HSR/ESR_EL2 indicates whether a data abort was
    generated by a read or a write instruction. For stage 2 data aborts
    generated by a stage 1 translation table walk (i.e. the actual page
    table access faults at EL2), the WnR bit therefore reports whether the
    instruction generating the walk was a load or a store, *not* whether the
    page table walker was reading or writing the entry.

    For page tables marked as read-only at stage 2 (e.g. due to KSM merging
    them with the tables from another guest), this could result in livelock,
    where a page table walk generated by a load instruction attempts to
    set the access flag in the stage 1 descriptor, but fails to trigger
    CoW in the host since only a read fault is reported.

    This patch modifies the arm64 kvm_vcpu_dabt_iswrite function to
    take into account stage 2 faults in stage 1 walks. Since DBM cannot be
    disabled at EL2 for CPUs that implement it, we assume that these faults
    are always causes by writes, avoiding the livelock situation at the
    expense of occasional, spurious CoWs.

    We could, in theory, do a bit better by checking the guest TCR
    configuration and inspecting the page table to see why the PTE faulted.
    However, I doubt this is measurable in practice, and the threat of
    livelock is real.

    Cc: Julien Grall <julien.grall@arm.com>
    Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
    Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
    Signed-off-by: Will Deacon <will.deacon@arm.com>
    [bwh: Backported to 3.16:
    - Keep using ESR_EL2_WNR in the first part of the condition
    - Adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    arch/arm64/include/asm/kvm_emulate.h | 11 ++++++-----
    1 file changed, 6 insertions(+), 5 deletions(-)

    --- a/arch/arm64/include/asm/kvm_emulate.h
    +++ b/arch/arm64/include/asm/kvm_emulate.h
    @@ -135,11 +135,6 @@ static inline bool kvm_vcpu_dabt_isvalid
    return !!(kvm_vcpu_get_hsr(vcpu) & ESR_EL2_ISV);
    }

    -static inline bool kvm_vcpu_dabt_iswrite(const struct kvm_vcpu *vcpu)
    -{
    - return !!(kvm_vcpu_get_hsr(vcpu) & ESR_EL2_WNR);
    -}
    -
    static inline bool kvm_vcpu_dabt_issext(const struct kvm_vcpu *vcpu)
    {
    return !!(kvm_vcpu_get_hsr(vcpu) & ESR_EL2_SSE);
    @@ -160,6 +155,12 @@ static inline bool kvm_vcpu_dabt_iss1tw(
    return !!(kvm_vcpu_get_hsr(vcpu) & ESR_EL2_S1PTW);
    }

    +static inline bool kvm_vcpu_dabt_iswrite(const struct kvm_vcpu *vcpu)
    +{
    + return !!(kvm_vcpu_get_hsr(vcpu) & ESR_EL2_WNR) ||
    + kvm_vcpu_dabt_iss1tw(vcpu); /* AF/DBM update */
    +}
    +
    static inline int kvm_vcpu_dabt_get_as(const struct kvm_vcpu *vcpu)
    {
    return 1 << ((kvm_vcpu_get_hsr(vcpu) & ESR_EL2_SAS) >> ESR_EL2_SAS_SHIFT);
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