Re: [PATCH 2/3] KVM: x86: Skip IN_GUEST_MODE vCPUs in kvm_vcpu_on_spin main loop

[Sean Christopherson <seanjc@google.com>]

On Sun, Feb 15, 2026, 76824143@qq.com wrote:
> From: zhanghao <zhanghao1@kylinos.cn>
> 
> Add a check in the kvm_vcpu_on_spin() main loop to skip vCPUs
> that are already running in guest mode.
> 
> Reduces unnecessary yield_to() calls and VM exits.
> 
> Signed-off-by: zhanghao <zhanghao1@kylinos.cn>
> ---
>  virt/kvm/kvm_main.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 476ecdb18bdd..663df3a121c8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4026,6 +4026,10 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
>  		vcpu = xa_load(&kvm->vcpu_array, idx);
>  		if (!READ_ONCE(vcpu->ready))
>  			continue;
> +
> +		if (READ_ONCE(vcpu->mode) == IN_GUEST_MODE)
> +			continue;

This should generally not happen, as vcpu->ready should only be true when a vCPU
is scheduled out.  Although it does look like there's a race in kvm_vcpu_wake_up()
where vcpu->ready could be left %true, e.g. if the task was delyed or preempted
after __kvm_vcpu_wake_up(), before the "WRITE_ONCE(vcpu->ready, true)".  Not sure
how best to handle that scenario.

> +
>  		if (kvm_vcpu_is_blocking(vcpu) && !vcpu_dy_runnable(vcpu))
>  			continue;
>  
> -- 
> 2.39.2
>