linux-kernel - Re: [PATCH 06/10] KVM: Fix last_boosted

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Message-ID: <aR-zxrYATZ4rZZjn@google.com>
Date: Thu, 20 Nov 2025 16:35:18 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Wanpeng Li <kernellwp@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Paolo Bonzini <pbonzini@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Juri Lelli <juri.lelli@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH 06/10] KVM: Fix last_boosted_vcpu index assignment bug

On Mon, Nov 10, 2025, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> From: Wanpeng Li <wanpengli@...cent.com>

Something might be off in your email scripts.  Speaking of email, mostly as an
FYI, your @tencent email was bouncing as of last year, and prompted commit
b018589013d6 ("MAINTAINERS: Drop Wanpeng Li as a Reviewer for KVM Paravirt support").

> In kvm_vcpu_on_spin(), the loop counter 'i' is incorrectly written to
> last_boosted_vcpu instead of the actual vCPU index 'idx'. This causes
> last_boosted_vcpu to store the loop iteration count rather than the
> vCPU index, leading to incorrect round-robin behavior in subsequent
> directed yield operations.
> 
> Fix this by using 'idx' instead of 'i' in the assignment.

Fixes: 7e513617da71 ("KVM: Rework core loop of kvm_vcpu_on_spin() to use a single for-loop")
Cc: stable@...r.kernel.org
Reviewed-by: Sean Christopherson <seanjc@...gle.com>

Please, please don't bury fixes like this in a large-ish series, especially in a
series that's going to be quite contentious and thus likely to linger on-list for
quite some time.  It's pretty much dumb luck on my end that I saw this.

That said, thank you for fixing my goof :-)

Paolo, do you want to grab this for 6.19?  Or just wait for 6.20?

> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  virt/kvm/kvm_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index b7a0ae2a7b20..cde1eddbaa91 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4026,7 +4026,7 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>  
>  		yielded = kvm_vcpu_yield_to(vcpu);
>  		if (yielded > 0) {
> -			WRITE_ONCE(kvm->last_boosted_vcpu, i);
> +			WRITE_ONCE(kvm->last_boosted_vcpu, idx);
>  			break;
>  		} else if (yielded < 0 && !--try) {
>  			break;
> -- 
> 2.43.0
>