Re: [PATCH] KVM: arm64: Keep need_db always true in vgic_v4_put() when emulating WFI

[Oliver Upton <oliver.upton@linux.dev>]

Hi Xiang,

Thanks for reporting this.

On Fri, Jul 07, 2023 at 04:55:45PM +0800, chenxiang wrote:
> When enabled GICv4.1 on Kunpeng 920 platform with 6.4 kernel (preemptible
> kernel), running a vm with 128 vcpus on 64 pcpu, sometimes vm is not booted
> successfully, and we find there is a situation that doorbell interrupt will
> not occur event if there is a pending interrupt.

Oh, that's no good.

> ---
>  arch/arm64/kvm/vgic/vgic-v4.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
> index c1c28fe..37152cf8 100644
> --- a/arch/arm64/kvm/vgic/vgic-v4.c
> +++ b/arch/arm64/kvm/vgic/vgic-v4.c
> @@ -343,6 +343,9 @@ int vgic_v4_put(struct kvm_vcpu *vcpu, bool need_db)
>  	if (!vgic_supports_direct_msis(vcpu->kvm) || !vpe->resident)
>  		return 0;
>  
> +	if (vcpu->stat.generic.blocking == 1)
> +		need_db = true;
> +

As I understand it, the issue really comes from the fact that @need_db
is always false when called from vgic_v3_put(). I'd rather we not
override the argument, as that could have unintended consequences in the
future.

You could also use the helper we already have for determining if a vCPU
is blocking, which we could use as a hint for requesting a doorbell
interrupt on sched out.

Putting the two comments together, I arrived at the following (untested)
diff. I don't have a GICv4 system on hand, sadly. Mind taking it for a
spin?

--
Thanks,
Oliver

diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index 93a47a515c13..8c743643b122 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -756,7 +756,7 @@ void vgic_v3_put(struct kvm_vcpu *vcpu)
 {
 	struct vgic_v3_cpu_if *cpu_if = &vcpu->arch.vgic_cpu.vgic_v3;
 
-	WARN_ON(vgic_v4_put(vcpu, false));
+	WARN_ON(vgic_v4_put(vcpu, kvm_vcpu_is_blocking(vcpu)));
 
 	vgic_v3_vmcr_sync(vcpu);