Re: vhost: multiple worker support

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

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?

For example, two competing users could sabotague each other by running
lots of work items on their competitor's vhost worker thread.