Marc Zyngier [off-list ref] writes:

Hi Vitaly,

On 2021-11-11 16:27, Vitaly Kuznetsov wrote:

quoted

It doesn't make sense to return the recommended maximum number of
vCPUs which exceeds the maximum possible number of vCPUs.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 arch/arm64/kvm/arm.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 7838e9fb693e..391dc7a921d5 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c

@@ -223,7 +223,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, 

long ext)
 		r = 1;
 		break;
 	case KVM_CAP_NR_VCPUS:
-		r = num_online_cpus();
+		if (kvm)
+			r = min_t(unsigned int, num_online_cpus(),
+				  kvm->arch.max_vcpus);
+		else
+			r = min_t(unsigned int, num_online_cpus(),
+				  kvm_arm_default_max_vcpus());
 		break;
 	case KVM_CAP_MAX_VCPUS:
 	case KVM_CAP_MAX_VCPU_ID:

This looks odd. This means that depending on the phase userspace is
in while initialising the VM, KVM_CAP_NR_VCPUS can return one thing
or the other.

For example, I create a VM on a 32 CPU system, NR_VCPUS says 32.
I create a GICv2 interrupt controller, it now says 8.

That's a change in behaviour that is visible by userspace

Yes, I realize this is a userspace visible change. The reason I suggest
it is that logically, it seems very odd that the maximum recommended
number of vCPUs (KVM_CAP_NR_VCPUS) can be higher, than the maximum
supported number of vCPUs (KVM_CAP_MAX_VCPUS). All userspaces which use
this information somehow should already contain some workaround for this
case. (maybe it's a rare one and nobody hit it yet or maybe there are no
userspaces using KVM_CAP_NR_VCPUS for anything besides complaining --
like QEMU).

I'd like KVM to be consistent across architectures and have the same
(similar) meaning for KVM_CAP_NR_VCPUS.

which I'm keen on avoiding. I'd rather have the kvm and !kvm cases
return the same thing.

Forgive me my (ARM?) ignorance but what would it be then? If we go for
min(num_online_cpus(), kvm_arm_default_max_vcpus()) in both cases, cat
this can still go above KVM_CAP_MAX_VCPUS after vGIC is created?

Thanks for the feedback!

-- 
Vitaly


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel