[PATCH v2] KVM: LAPIC: Enable timer posted-interrupt when mwait/hlt is advertised

[PATCH v2] KVM: LAPIC: Enable timer posted-interrupt when mwait/hlt is advertised

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

As commit 0c5f81dad46 (KVM: LAPIC: Inject timer interrupt via posted interrupt) 
mentioned that the host admin should well tune the guest setup, so that vCPUs 
are placed on isolated pCPUs, and with several pCPUs surplus for *busy* housekeeping.
It is better to disable mwait/hlt/pause vmexits to keep the vCPUs in non-root 
mode. However, we may isolate pCPUs for other purpose like DPDK or we can make 
some guests isolated and others not, we may lose vmx preemption timer/timer fastpath 
due to not well tuned setup, and the checking in kvm_can_post_timer_interrupt() 
is not enough. Let's guarantee mwait/hlt is advertised before enabling posted-interrupt 
interrupt. vmx preemption timer/timer fastpath can continue to work if both of them 
are not advertised.

Reported-by: Aili Yao <yaoaili@kingsoft.com>
Cc: Aili Yao <yaoaili@kingsoft.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
v1 -> v2:
 * also check kvm_hlt_in_guest since sometime mwait is disabled on host

 arch/x86/kvm/lapic.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index f206fc3..fdb7c81 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -113,7 +113,8 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
 
 static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
 {
-	return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
+	return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
+		(kvm_mwait_in_guest(vcpu->kvm) || kvm_hlt_in_guest(vcpu->kvm));
 }
 
 bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
-- 
2.7.4

Re: [PATCH v2] KVM: LAPIC: Enable timer posted-interrupt when mwait/hlt is advertised

[Sean Christopherson]

On Thu, Jan 06, 2022, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> As commit 0c5f81dad46 (KVM: LAPIC: Inject timer interrupt via posted interrupt) 
> mentioned that the host admin should well tune the guest setup, so that vCPUs 
> are placed on isolated pCPUs, and with several pCPUs surplus for *busy* housekeeping.
> It is better to disable mwait/hlt/pause vmexits to keep the vCPUs in non-root 
> mode. However, we may isolate pCPUs for other purpose like DPDK or we can make 
> some guests isolated and others not, we may lose vmx preemption timer/timer fastpath 
> due to not well tuned setup, and the checking in kvm_can_post_timer_interrupt() 
> is not enough. Let's guarantee mwait/hlt is advertised before enabling posted-interrupt 
> interrupt. vmx preemption timer/timer fastpath can continue to work if both of them 
> are not advertised.
> 
> Reported-by: Aili Yao <yaoaili@kingsoft.com>
> Cc: Aili Yao <yaoaili@kingsoft.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

Re: [PATCH v2] KVM: LAPIC: Enable timer posted-interrupt when mwait/hlt is advertised

[Wanpeng Li]

kindly ping, :)
On Thu, 6 Jan 2022 at 20:21, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> From: Wanpeng Li <wanpengli@tencent.com>
>
> As commit 0c5f81dad46 (KVM: LAPIC: Inject timer interrupt via posted interrupt)
> mentioned that the host admin should well tune the guest setup, so that vCPUs
> are placed on isolated pCPUs, and with several pCPUs surplus for *busy* housekeeping.
> It is better to disable mwait/hlt/pause vmexits to keep the vCPUs in non-root
> mode. However, we may isolate pCPUs for other purpose like DPDK or we can make
> some guests isolated and others not, we may lose vmx preemption timer/timer fastpath
> due to not well tuned setup, and the checking in kvm_can_post_timer_interrupt()
> is not enough. Let's guarantee mwait/hlt is advertised before enabling posted-interrupt
> interrupt. vmx preemption timer/timer fastpath can continue to work if both of them
> are not advertised.
>
> Reported-by: Aili Yao <yaoaili@kingsoft.com>
> Cc: Aili Yao <yaoaili@kingsoft.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> v1 -> v2:
>  * also check kvm_hlt_in_guest since sometime mwait is disabled on host
>
>  arch/x86/kvm/lapic.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index f206fc3..fdb7c81 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -113,7 +113,8 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
>
>  static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
>  {
> -       return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> +       return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> +               (kvm_mwait_in_guest(vcpu->kvm) || kvm_hlt_in_guest(vcpu->kvm));
>  }
>
>  bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> --
> 2.7.4
>