Re: [PATCH V3 11/11] vhost: allow userspace to create workers

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On Wed, Oct 27, 2021 at 10:55:04AM +0800, Jason Wang wrote:
> On Tue, Oct 26, 2021 at 11:45 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> >
> > On Tue, Oct 26, 2021 at 01:37:14PM +0800, Jason Wang wrote:
> > >
> > > 在 2021/10/22 下午1:19, Mike Christie 写道:
> > > > This patch allows userspace to create workers and bind them to vqs. You
> > > > can have N workers per dev and also share N workers with M vqs.
> > > >
> > > > Signed-off-by: Mike Christie <michael.christie@xxxxxxxxxx>
> > >
> > >
> > > A question, who is the best one to determine the binding? Is it the VMM
> > > (Qemu etc) or the management stack? If the latter, it looks to me it's
> > > better to expose this via sysfs?
> >
> > A few options that let the management stack control vhost worker CPU
> > affinity:
> >
> > 1. The management tool opens the vhost device node, calls
> >    ioctl(VHOST_SET_VRING_WORKER), sets up CPU affinity, and then passes
> >    the fd to the VMM. In this case the VMM is still able to call the
> >    ioctl, which may be undesirable from an attack surface perspective.
> 
> Yes, and we can't do post or dynamic configuration afterwards after
> the VM is launched?

Yes, at least it's a little risky for the management stack to keep the
vhost fd open and make ioctl calls while the VMM is using it.

> >
> > 2. The VMM calls ioctl(VHOST_SET_VRING_WORKER) itself and the management
> >    tool queries the vq:worker details from the VMM (e.g. a new QEMU QMP
> >    query-vhost-workers command similar to query-iothreads). The
> >    management tool can then control CPU affinity on the vhost worker
> >    threads.
> >
> >    (This is how CPU affinity works in QEMU and libvirt today.)
> 
> Then we also need a "bind-vhost-workers" command.

The VMM doesn't but the management tool does.

Stefan

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