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Subject: Re: [PATCH 01/14] KVM: s390: Ensure kvm_arch_no_poll() is read once
 when blocking vCPU
To:     Sean Christopherson <seanjc@google.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Janosch Frank <frankja@linux.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Cc:     James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Jing Zhang <jingzhangos@google.com>
References: <20210925005528.1145584-1-seanjc@google.com>
 <20210925005528.1145584-2-seanjc@google.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <01fa2462-6652-7206-5ef3-bacb3452b4c8@de.ibm.com>
Date:   Mon, 27 Sep 2021 08:54:28 +0200
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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@de.ibm.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.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) {
>