KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU

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")
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index b943a58..9653e40 100644 (file)
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3403,7 +3403,7 @@ 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 e9990c4..a26b069 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3284,6 +3284,7 @@ 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;
@@ -3291,7 +3292,7 @@ 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;
@@ -3346,7 +3347,7 @@ out:
        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) {