Re: [PATCH 01/14] KVM: s390: Ensure kvm_arch_no_poll() is read once when blocking vCPU

[Christian Borntraeger]

Am 25.09.21 um 02:55 schrieb Sean Christopherson:
> 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@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>

> ---
>   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) {