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    Subject[PATCH RFC 1/6] KVM: fix guest_mode optimization in kvm_make_all_cpus_request()
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    We have kvm_arch_vcpu_should_kick() to decide whether the target cpu
    needs to be kicked. The previous condition was wrong, because
    architectures that don't use vcpu->mode would not get interrupts and
    also suboptimal, because it sent IPI in cases where none was necessary.

    The situation is even more convoluted. MIPS and POWERPC return 1 from
    kvm_arch_vcpu_should_kick(), but implement vcpu->mode for some reason,
    so now they might kick uselessly. This is not a huge problem.

    s390, on the other hand, never changed vcpu->mode, so it would always be
    OUTSIDE_GUEST_MODE before and therefore didn't send IPIs.
    I don't see a reason why s390 had kvm_make_all_cpus_request() that did
    nothing but set the bit in vcpu->request, so the new behavior seems
    better.

    Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
    ---
    arch/s390/kvm/kvm-s390.c | 4 +---
    virt/kvm/kvm_main.c | 2 +-
    2 files changed, 2 insertions(+), 4 deletions(-)

    diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
    index fd6cd05bb6a7..45b6d9ca5d24 100644
    --- a/arch/s390/kvm/kvm-s390.c
    +++ b/arch/s390/kvm/kvm-s390.c
    @@ -2130,9 +2130,7 @@ static void kvm_gmap_notifier(struct gmap *gmap, unsigned long start,

    int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
    {
    - /* kvm common code refers to this, but never calls it */
    - BUG();
    - return 0;
    + return 1;
    }

    static int kvm_arch_vcpu_ioctl_get_one_reg(struct kvm_vcpu *vcpu,
    diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
    index f489167839c4..2389e9c41cd2 100644
    --- a/virt/kvm/kvm_main.c
    +++ b/virt/kvm/kvm_main.c
    @@ -187,7 +187,7 @@ bool kvm_make_all_cpus_request(struct kvm *kvm, unsigned int req)
    smp_mb__after_atomic();

    if (cpus != NULL && cpu != -1 && cpu != me &&
    - kvm_vcpu_exiting_guest_mode(vcpu) != OUTSIDE_GUEST_MODE)
    + kvm_arch_vcpu_should_kick(vcpu))
    cpumask_set_cpu(cpu, cpus);
    }
    if (unlikely(cpus == NULL))
    --
    2.12.0
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