Re: [PATCH v3 3/3] KVM: x86: Drop superfluous caching of KVM_ASYNC_PF_SEND_ALWAYS

[Binbin Wu]

On 4/7/2026 6:53 AM, Sean Christopherson wrote:
> Drop kvm_vcpu_arch.send_always and instead use msr_en_val as the source of
> truth to reduce the probability of operating on stale data.  This fixes
> flaws where KVM fails to update send_always when APF is explicitly
> disabled by the guest or implicitly disabled by KVM on INIT.  Absent other
> bugs, the flaws are benign as KVM *shouldn't* consume send_always when PV
> APF support is disabled.
> 
> Simply delete the field, as there's zero benefit to maintaining a separate
> "cache" of the state.
> 
> Opportunistically turn the enabled vs. disabled logic at the end of
> kvm_pv_enable_async_pf() into an if-else instead of using an early return,
> e.g. so that it's more obvious that both paths are "success" paths.

Nit:
Drop "e.g." ?

> 
> Fixes: 6adba5274206 ("KVM: Let host know whether the guest can handle async PF in non-userspace context.")
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> ---
>  arch/x86/include/asm/kvm_host.h |  1 -
>  arch/x86/kvm/x86.c              | 12 ++++--------
>  2 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index fae1f4aeca5a..2a6906597637 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1038,7 +1038,6 @@ struct kvm_vcpu_arch {
>  		u16 vec;
>  		u32 id;
>  		u32 host_apf_flags;
> -		bool send_always;
>  		bool pageready_pending;
>  	} apf;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4632222a5d1c..e24877353f17 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3659,16 +3659,12 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
>  
>  	vcpu->arch.apf.msr_en_val = data;
>  
> -	if (!__kvm_pv_async_pf_enabled(data)) {
> +	if (__kvm_pv_async_pf_enabled(data)) {
> +		kvm_async_pf_wakeup_all(vcpu);
> +	} else {
>  		kvm_clear_async_pf_completion_queue(vcpu);
>  		kvm_async_pf_hash_reset(vcpu);
> -		return 0;
>  	}
> -
> -	vcpu->arch.apf.send_always = (data & KVM_ASYNC_PF_SEND_ALWAYS);
> -
> -	kvm_async_pf_wakeup_all(vcpu);
> -
>  	return 0;
>  }
>  
> @@ -14025,7 +14021,7 @@ static bool kvm_can_deliver_async_pf(struct kvm_vcpu *vcpu)
>  	if (!kvm_pv_async_pf_enabled(vcpu))
>  		return false;
>  
> -	if (!vcpu->arch.apf.send_always &&
> +	if (!(vcpu->arch.apf.msr_en_val & KVM_ASYNC_PF_SEND_ALWAYS) &&
>  	    (vcpu->arch.guest_state_protected || !kvm_x86_call(get_cpl)(vcpu)))
>  		return false;
>