linux-kernel - Re: [v5 3/3] KVM: s390: don't setup dummy routing when KVM_CREATE

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Message-ID: <ZmcnlPcplno-toU4@google.com>
Date: Mon, 10 Jun 2024 09:19:32 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yi Wang <up2wing@...il.com>
Cc: pbonzini@...hat.com, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, wanpengli@...cent.com, 
	foxywang@...cent.com, oliver.upton@...ux.dev, maz@...nel.org, 
	anup@...infault.org, atishp@...shpatra.org, borntraeger@...ux.ibm.com, 
	frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, weijiang.yang@...el.com
Subject: Re: [v5 3/3] KVM: s390: don't setup dummy routing when KVM_CREATE_IRQCHIP

On Mon, May 06, 2024, Yi Wang wrote:
> From: Yi Wang <foxywang@...cent.com>
> 
> As we have setup empty irq routing in kvm_create_vm(), there's
> no need to setup dummy routing when KVM_CREATE_IRQCHIP.
> 
> Signed-off-by: Yi Wang <foxywang@...cent.com>
> ---
>  arch/s390/kvm/kvm-s390.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 5147b943a864..ba7fd39bcbf4 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2998,14 +2998,7 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  		break;
>  	}
>  	case KVM_CREATE_IRQCHIP: {
> -		struct kvm_irq_routing_entry routing;
> -
> -		r = -EINVAL;
> -		if (kvm->arch.use_irqchip) {
> -			/* Set up dummy routing. */
> -			memset(&routing, 0, sizeof(routing));
> -			r = kvm_set_irq_routing(kvm, &routing, 0, 0);
> -		}
> +		r = 0;

This is wrong, KVM_CREATE_IRQCHIP should fail with -EINVAL if kvm->arch.use_irqchip
is false.

There's also a functional change here, though I highly doubt it negatively affects
userspace.  Nothing in s390 prevents invoking KVM_CREATE_IRQCHIP after
KVM_SET_GSI_ROUTING, so userspace could very theoretically use KVM_CREATE_IRQCHIP
to reset to empty IRQ routing.

Christian, if it works for you, I'll massage it to this when applying.

--
From: Yi Wang <foxywang@...cent.com>
Date: Mon, 6 May 2024 18:17:51 +0800
Subject: [PATCH] KVM: s390: Don't re-setup dummy routing when
 KVM_CREATE_IRQCHIP

Now that KVM sets up empty irq routing in kvm_create_vm(), there's
no need to setup dummy routing when KVM_CREATE_IRQCHIP.

Note, userspace could very theoretically use KVM_CREATE_IRQCHIP after
KVM_SET_GSI_ROUTING to reset to empty IRQ routing, but it's extremely
unlikely any VMM does that, e.g. the main reason s390 does anything for
KVM_CREATE_IRQCHIP is to that s390 doesn't need to be special cased by the
VMM.

Signed-off-by: Yi Wang <foxywang@...cent.com>
Acked-by: Christian Borntraeger <borntraeger@...ux.ibm.com>
Link: https://lore.kernel.org/r/20240506101751.3145407-4-foxywang@tencent.com
[sean: keep use_irqchip check, call out KVM_SET_GSI_ROUTING impact]
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/s390/kvm/kvm-s390.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 82e9631cd9ef..4641083ee100 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -2996,14 +2996,9 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		break;
 	}
 	case KVM_CREATE_IRQCHIP: {
-		struct kvm_irq_routing_entry routing;
-
 		r = -EINVAL;
-		if (kvm->arch.use_irqchip) {
-			/* Set up dummy routing. */
-			memset(&routing, 0, sizeof(routing));
-			r = kvm_set_irq_routing(kvm, &routing, 0, 0);
-		}
+		if (kvm->arch.use_irqchip)
+			r = 0;
 		break;
 	}
 	case KVM_SET_DEVICE_ATTR: {

base-commit: 9a859becf1b7f6879466e8c0ebee492b236f2080
--