Re: vhost: multiple worker support

On 6/3/21 9:37 AM, Stefan Hajnoczi wrote:

On Tue, May 25, 2021 at 01:05:51PM -0500, Mike Christie wrote:

quoted

The following patches apply over linus's tree or mst's vhost branch
and my cleanup patchset:

https://lists.linuxfoundation.org/pipermail/virtualization/2021-May/054354.html

These patches allow us to support multiple vhost workers per device. I
ended up just doing Stefan's original idea where userspace has the
kernel create a worker and we pass back the pid. This has the benefit
over the workqueue and userspace thread approach where we only have
one'ish code path in the kernel during setup to detect old tools. The
main IO paths and device/vq setup/teardown paths all use common code.

The kernel patches here allow us to then do N workers device and also
share workers across devices.

I've also included a patch for qemu so you can get an idea of how it
works. If we are ok with the kernel code then I'll break that up into
a patchset and send to qemu-devel.

It seems risky to allow userspace process A to "share" a vhost worker
thread with userspace process B based on a matching pid alone. Should
they have ptrace_may_access() or similar?

I'm not sure. I already made it a little restrictive in this posting, but
it may not be enough depending on what's possible and what we want to allow.

Right now to share a worker the userspace process doing the
VHOST_SET_VRING_WORKER ioctl has to be the owner. Before we do a
VHOST_SET_VRING_WORKER, vhost_dev_ioctl calls vhost_dev_check_owner,
so we will fail if 2 random processes try to share.

So we can share a worker across a vhost-dev's N virtqueues or share a
worker with multiple vhost devs and their virtqueues, but the devs have
to be managed by the same VM/qemu process. If that's all we want to support,
then is the owner check enough?

If we want to share workers across VMs then I think we definitely want
something like ptrace_may_access.