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    Subject[PATCH 01/14] KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU
    From
    Wrap s390's halt_poll_max_steal with READ_ONCE and snapshot the result of
    kvm_arch_no_poll() in kvm_vcpu_block() to avoid a mostly-theoretical,
    largely benign bug on s390 where the result of kvm_arch_no_poll() could
    change due to userspace modifying halt_poll_max_steal while the vCPU is
    blocking. The bug is largely benign as it will either cause KVM to skip
    updating halt-polling times (no_poll toggles false=>true) or to update
    halt-polling times with a slightly flawed block_ns.

    Note, READ_ONCE is unnecessary in the current code, add it in case the
    arch hook is ever inlined, and to provide a hint that userspace can
    change the param at will.

    Fixes: 8b905d28ee17 ("KVM: s390: provide kvm_arch_no_poll function")
    Cc: Christian Borntraeger <borntraeger@de.ibm.com>
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    ---
    arch/s390/kvm/kvm-s390.c | 2 +-
    virt/kvm/kvm_main.c | 5 +++--
    2 files changed, 4 insertions(+), 3 deletions(-)

    diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
    index 6a6dd5e1daf6..7cabe6778b1b 100644
    --- a/arch/s390/kvm/kvm-s390.c
    +++ b/arch/s390/kvm/kvm-s390.c
    @@ -3446,7 +3446,7 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
    {
    /* do not poll with more than halt_poll_max_steal percent of steal time */
    if (S390_lowcore.avg_steal_timer * 100 / (TICK_USEC << 12) >=
    - halt_poll_max_steal) {
    + READ_ONCE(halt_poll_max_steal)) {
    vcpu->stat.halt_no_poll_steal++;
    return true;
    }
    diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
    index 191dac6b1bed..768a4cbb26a6 100644
    --- a/virt/kvm/kvm_main.c
    +++ b/virt/kvm/kvm_main.c
    @@ -3213,6 +3213,7 @@ update_halt_poll_stats(struct kvm_vcpu *vcpu, u64 poll_ns, bool waited)
    */
    void kvm_vcpu_block(struct kvm_vcpu *vcpu)
    {
    + bool halt_poll_allowed = !kvm_arch_no_poll(vcpu);
    ktime_t start, cur, poll_end;
    bool waited = false;
    u64 block_ns;
    @@ -3220,7 +3221,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
    kvm_arch_vcpu_blocking(vcpu);

    start = cur = poll_end = ktime_get();
    - if (vcpu->halt_poll_ns && !kvm_arch_no_poll(vcpu)) {
    + if (vcpu->halt_poll_ns && halt_poll_allowed) {
    ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);

    ++vcpu->stat.generic.halt_attempted_poll;
    @@ -3275,7 +3276,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
    update_halt_poll_stats(
    vcpu, ktime_to_ns(ktime_sub(poll_end, start)), waited);

    - if (!kvm_arch_no_poll(vcpu)) {
    + if (halt_poll_allowed) {
    if (!vcpu_valid_wakeup(vcpu)) {
    shrink_halt_poll_ns(vcpu);
    } else if (vcpu->kvm->max_halt_poll_ns) {
    --
    2.33.0.685.g46640cef36-goog
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