Re: [PATCH][kvmtool] kvm: Request VM specific limits instead of system-wide ones

["André Przywara" <andre.przywara@arm.com>]

On 27/04/2020 15:17, Marc Zyngier wrote:
Hi,

> On arm64, the maximum number of vcpus is constrained by the type
> of interrupt controller that has been selected (GICv2 imposes a
> limit of 8 vcpus, while GICv3 currently has a limit of 512).
> 
> It is thus important to request this limit on the VM file descriptor
> rather than on the one that corresponds to /dev/kvm, as the latter
> is likely to return something that doesn't take the constraints into
> account.

That sounds reasonable, but I fail to find any distinction in the kernel
code. We don't make any difference between the VM or the system FD in
the ioctl handler for those two extensions. For arm64 we always return
max. 512 (max VCPUs on GICv3), and number of online host cores for the
recommended value. For arm there was a distinction between GICv3 support
compiled in or not, but otherwise the same constant values returned.
Quickly tested on Juno and N1SDP, the ioctls return the same expected
values, regardless of sys_fd vs vm_fd.

So what am I missing here? Is this for some older or even newer kernels?

Cheers,
Andre.

> 
> Reported-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  kvm.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kvm.c b/kvm.c
> index e327541..3d5173d 100644
> --- a/kvm.c
> +++ b/kvm.c
> @@ -406,7 +406,7 @@ int kvm__recommended_cpus(struct kvm *kvm)
>  {
>  	int ret;
>  
> -	ret = ioctl(kvm->sys_fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
> +	ret = ioctl(kvm->vm_fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
>  	if (ret <= 0)
>  		/*
>  		 * api.txt states that if KVM_CAP_NR_VCPUS does not exist,
> @@ -421,7 +421,7 @@ int kvm__max_cpus(struct kvm *kvm)
>  {
>  	int ret;
>  
> -	ret = ioctl(kvm->sys_fd, KVM_CHECK_EXTENSION, KVM_CAP_MAX_VCPUS);
> +	ret = ioctl(kvm->vm_fd, KVM_CHECK_EXTENSION, KVM_CAP_MAX_VCPUS);
>  	if (ret <= 0)
>  		ret = kvm__recommended_cpus(kvm);
>  
> 

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