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    Subject[PATCH 5.2 175/215] KVM: PPC: Book3S HV: Always save guest pmu for guest capable of nesting
    Date
    From: Suraj Jitindar Singh <sjitindarsingh@gmail.com>

    commit 63279eeb7f93abb1692573c26f1e038e1a87358b upstream.

    The performance monitoring unit (PMU) registers are saved on guest
    exit when the guest has set the pmcregs_in_use flag in its lppaca, if
    it exists, or unconditionally if it doesn't. If a nested guest is
    being run then the hypervisor doesn't, and in most cases can't, know
    if the PMU registers are in use since it doesn't know the location of
    the lppaca for the nested guest, although it may have one for its
    immediate guest. This results in the values of these registers being
    lost across nested guest entry and exit in the case where the nested
    guest was making use of the performance monitoring facility while it's
    nested guest hypervisor wasn't.

    Further more the hypervisor could interrupt a guest hypervisor between
    when it has loaded up the PMU registers and it calling H_ENTER_NESTED
    or between returning from the nested guest to the guest hypervisor and
    the guest hypervisor reading the PMU registers, in
    kvmhv_p9_guest_entry(). This means that it isn't sufficient to just
    save the PMU registers when entering or exiting a nested guest, but
    that it is necessary to always save the PMU registers whenever a guest
    is capable of running nested guests to ensure the register values
    aren't lost in the context switch.

    Ensure the PMU register values are preserved by always saving their
    value into the vcpu struct when a guest is capable of running nested
    guests.

    This should have minimal performance impact however any impact can be
    avoided by booting a guest with "-machine pseries,cap-nested-hv=false"
    on the qemu commandline.

    Fixes: 95a6432ce903 ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests")
    Cc: stable@vger.kernel.org # v4.20+
    Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20190703012022.15644-1-sjitindarsingh@gmail.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/kvm/book3s_hv.c | 2 ++
    1 file changed, 2 insertions(+)

    --- a/arch/powerpc/kvm/book3s_hv.c
    +++ b/arch/powerpc/kvm/book3s_hv.c
    @@ -3654,6 +3654,8 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu
    vcpu->arch.vpa.dirty = 1;
    save_pmu = lp->pmcregs_in_use;
    }
    + /* Must save pmu if this guest is capable of running nested guests */
    + save_pmu |= nesting_enabled(vcpu->kvm);

    kvmhv_save_guest_pmu(vcpu, save_pmu);


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