Virtualization - Dec 2020 - [RFC PATCH 5/8] vhost: allow userspace to bind vqs to CPUs

On 2020/12/5 ??12:32, Mike Christie wrote:
> On 12/4/20 2:09 AM, Jason Wang wrote:
>>
>> On 2020/12/4 ??3:56, Mike Christie wrote:
>>> +static long vhost_vring_set_cpu(struct vhost_dev *d, struct 
>>> vhost_virtqueue *vq,
>>> +??????????????? void __user *argp)
>>> +{
>>> +??? struct vhost_vring_state s;
>>> +??? int ret = 0;
>>> +
>>> +??? if (vq->private_data)
>>> +??????? return -EBUSY;
>>> +
>>> +??? if (copy_from_user(&s, argp, sizeof s))
>>> +??????? return -EFAULT;
>>> +
>>> +??? if (s.num == -1) {
>>> +??????? vq->cpu = s.num;
>>> +??????? return 0;
>>> +??? }
>>> +
>>> +??? if (s.num >= nr_cpu_ids)
>>> +??????? return -EINVAL;
>>> +
>>> +??? if (!d->ops || !d->ops->get_workqueue)
>>> +??????? return -EINVAL;
>>> +
>>> +??? if (!d->wq)
>>> +??????? d->wq = d->ops->get_workqueue();
>>> +??? if (!d->wq)
>>> +??????? return -EINVAL;
>>> +
>>> +??? vq->cpu = s.num;
>>> +??? return ret;
>>> +}
>>
>>
>> So one question here. Who is in charge of doing this set_cpu? Note 
>> that?sched_setaffinity(2) requires CAP_SYS_NICE to work, so I wonder 
>> whether or not it's legal for unprivileged Qemu to do this.
>
>
> I was having qemu do it when it's setting up the vqs since it had the 
> info there already.
>
> Is it normally the tool that makes calls into qemu that does the 
> operations that require CAP_SYS_NICE? 

My understanding is that it only matter scheduling. And this patch wants 
to change the affinity which should check that capability.

> If so, then I see the interface needs to be changed.

Actually, if I read this patch correctly it requires e.g qemu to make 
the decision instead of the management layer. This may bring some 
troubles to for e.g the libvirt emulatorpin[1] implementation.

Thanks

[1] 
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/virtualization_tuning_and_optimization_guide/sect-virtualization_tuning_optimization_guide-numa-numa_and_libvirt