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SubjectRe: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU
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On 19/10/21 19:34, Sean Christopherson wrote:
> The intent of the extra check was to avoid the locked instruction that comes with
> disabling preemption via rcu_read_lock(). But thinking more, the extra op should
> be little more than a basic arithmetic operation in the grand scheme on modern x86
> since the cache line is going to be locked and written no matter what, either
> immediately before or immediately after.

There should be no locked instructions unless you're using
PREEMPT_RT/PREEMPT_RCU, no? The preempt_disable count is in a percpu
variable.

>
> + /*
> + * Avoid the moderately expensive "should kick" operation if this pCPU
> + * is currently running the target vCPU, in which case it's a KVM bug
> + * if the vCPU is in the inner run loop.
> + */
> + if (vcpu == __this_cpu_read(kvm_running_vcpu) &&
> + !WARN_ON_ONCE(vcpu->mode == IN_GUEST_MODE))
> + goto out;
> +

It should not even be a problem if vcpu->mode == IN_GUEST_MODE, you just
set it to EXITING_GUEST_MODE without even the need for atomic_cmpxchg.
I'll send a few patches out, since I think I found some related issues.

Paolo

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