Patch "KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

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

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-s390-ensure-kvm_arch_no_poll-is-read-once-when-b.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 2274bce3b02d3be39e1413342e82bee1229f01b7
Author: Sean Christopherson <seanjc@xxxxxxxxxx>
Date:   Fri Oct 8 19:11:56 2021 -0700

    KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU
    
    [ Upstream commit 6f390916c4fb359507d9ac4bf1b28a4f8abee5c0 ]
    
    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@xxxxxxxxxx>
    Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
    Message-Id: <20211009021236.4122790-4-seanjc@xxxxxxxxxx>
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 9a8c086528f56..402597f9d0505 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3447,7 +3447,7 @@ 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 13aff136e6eef..f8b42e19bc775 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3222,6 +3222,7 @@ 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;
@@ -3229,7 +3230,7 @@ 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;
@@ -3284,7 +3285,7 @@ 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) {