Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq

[Stefan Hajnoczi]

On Thu, Nov 19, 2020 at 4:43 PM Mike Christie
<michael.christie@oracle.com> wrote:
>
> On 11/19/20 10:24 AM, Stefan Hajnoczi wrote:
> > On Thu, Nov 19, 2020 at 4:13 PM Mike Christie
> > <michael.christie@oracle.com> wrote:
> >>
> >> On 11/19/20 8:46 AM, Michael S. Tsirkin wrote:
> >>> On Wed, Nov 18, 2020 at 11:31:17AM +0000, Stefan Hajnoczi wrote:
> > struct vhost_run_worker_info {
> >      struct timespec *timeout;
> >      sigset_t *sigmask;
> >
> >      /* List of virtqueues to process */
> >      unsigned nvqs;
> >      unsigned vqs[];
> > };
> >
> > /* This blocks until the timeout is reached, a signal is received, or
> > the vhost device is destroyed */
> > int ret = ioctl(vhost_fd, VHOST_RUN_WORKER, &info);
> >
> > As you can see, userspace isn't involved with dealing with the
> > requests. It just acts as a thread donor to the vhost driver.
> >
> > We would want the VHOST_RUN_WORKER calls to be infrequent to avoid the
> > penalty of switching into the kernel, copying in the arguments, etc.
>
> I didn't get this part. Why have the timeout? When the timeout expires,
> does userspace just call right back down to the kernel or does it do
> some sort of processing/operation?
>
> You could have your worker function run from that ioctl wait for a
> signal or a wake up call from the vhost_work/poll functions.

An optional timeout argument is common in blocking interfaces like
poll(2), recvmmsg(2), etc.

Although something can send a signal to the thread instead,
implementing that in an application is more awkward than passing a
struct timespec.

Compared to other blocking calls we don't expect
ioctl(VHOST_RUN_WORKER) to return soon, so maybe the timeout will
rarely be used and can be dropped from the interface.

BTW the code I posted wasn't a carefully thought out proposal :). The
details still need to be considered and I'm going to be offline for
the next week so maybe someone else can think it through in the
meantime.

Stefan