linux-kernel - Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

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Date:   Wed, 20 Oct 2021 12:33:31 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 3/3] KVM: vCPU kick tax cut for running vCPU

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