On Fri, Nov 12, 2021 at 10:51:10AM +0100, Vitaly Kuznetsov wrote:

Marc Zyngier [off-list ref] writes:

quoted

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.

KVM_CAP_NR_VCPUS seems pretty useless if we just want to tell userspace
the same thing it can get with _SC_NPROCESSORS_ONLN. In fact, if userspace
knows something we don't about the future onlining of some pcpus, then
maybe userspace would prefer to check _SC_NPROCESSORS_CONF.

quoted

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?

So the GIC version case looks like the type of thing that could make
KVM_CAP_NR_VCPUS useful, i.e. being able to tell userspace a maximum
number of vcpus supported for a given configuration. However, even
in that case the concept of "recommended" number doesn't make sense,
because, for the GICv2 example, a VM cannot configure more than 8 VCPUs,
so it's a real limit, not a recommendation. Maybe KVM_CAP_NR_VCPUS should
just be left alone, but deprecated, and, if there's need, a new CAP could
be created for a config-vcpu-max.

Thanks,
drew


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