Hi,
On 4/27/20 3:44 PM, Alexandru Elisei wrote:
> Hi,
>
> On 4/27/20 3:17 PM, Marc Zyngier wrote:
>> 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.
>>
>> Reported-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
>> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
>> ---
>> 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);
>>
> I've checked that gic__create comes before the call kvm__recommended_capus:
> gic__create is in core_init (called via kvm__init->kvm_arch_init), and
> kvm__recommended_cpus is in base_init (called via kvm__cpu_init ->
> kvm__{recommended,max}_cpus).
>
> The KVM api documentation states that KVM_CHECK_EXTENSION is available for the vm
> fd only if the system capability KVM_CAP_CHECK_EXTENSION_VM is present. kvmtool
> already has a function for checking extensions on the vm fd, it's called
> kvm__supports_vm_extension. Can we use that instead of doing the ioctl directly on
> the vm fd?
Scratch that, kvm__supports_vm_extension returns a bool, not an int. How about we
write kvm__check_vm_extension that returns an int, and kvm__supports_vm_extension
calls it?
>
> Thanks,
> Alex
