Re: [PATCH 1/3] KVM: x86: Enhance kvm_vcpu_eligible_for_directed_yield to detect golden targets

[Sean Christopherson <seanjc@google.com>]

On Sun, Feb 15, 2026, 76824143@qq.com wrote:
> From: zhanghao <zhanghao1@kylinos.cn>
> 
> Detect "golden targets" - vCPUs that are preempted and ready.
> These are ideal yield targets as they can be immediately scheduled.
> 
> This check reduces unnecessary yield attempts to vCPUs that are
> unlikely to benefit from directed yield.
> 
> Signed-off-by: zhanghao <zhanghao1@kylinos.cn>
> ---
>  virt/kvm/kvm_main.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 61dca8d37abc..476ecdb18bdd 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3927,6 +3927,9 @@ static bool kvm_vcpu_eligible_for_directed_yield(struct kvm_vcpu *vcpu)
>  #ifdef CONFIG_HAVE_KVM_CPU_RELAX_INTERCEPT
>  	bool eligible;
>  
> +	if (READ_ONCE(vcpu->preempted) && READ_ONCE(vcpu->mode) == IN_GUEST_MODE)

This is nonsensical.  It should be impossible for a vCPU to be preempted while
IN_GUEST_MODE is true.  Even if a host IRQ arrives while the guest is active,
KVM should set vcpu->mode back to OUTSIDE_GUEST_MODE prior to servicing the IRQ.

Even more confusing, the next patch explicitly rejects IN_GUEST_MODE vCPUs from
kvm_vcpu_on_spin().

> +		return true;
> +
>  	eligible = !vcpu->spin_loop.in_spin_loop ||
>  		    vcpu->spin_loop.dy_eligible;
>  
> -- 
> 2.39.2
>