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

[Stefan Hajnoczi]

On Tue, Dec 01, 2020 at 02:45:18PM +0100, Stefano Garzarella wrote:
> On Tue, Dec 01, 2020 at 12:59:43PM +0000, Stefan Hajnoczi wrote:
> > On Fri, Nov 20, 2020 at 07:31:08AM -0500, Michael S. Tsirkin wrote:
> > > On Fri, Nov 20, 2020 at 08:45:49AM +0000, Stefan Hajnoczi wrote:
> > > > On Thu, Nov 19, 2020 at 5:08 PM Stefan Hajnoczi <stefanha@gmail.com> 
> > > > wrote:
> > > > >
> > > > > 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.
> > > >
> > > > One final thought before I'm offline for a week. If
> > > > ioctl(VHOST_RUN_WORKER) is specific to a single vhost device instance
> > > > then it's hard to support poll-mode (busy waiting) workers because
> > > > each device instance consumes a whole CPU. If we stick to an interface
> > > > where the kernel manages the worker threads then it's easier to share
> > > > workers between devices for polling.
> > > 
> > > 
> > > Yes that is the reason vhost did its own reason in the first place.
> > > 
> > > 
> > > I am vaguely thinking about poll(2) or a similar interface,
> > > which can wait for an event on multiple FDs.
> > 
> > I can imagine how using poll(2) would work from a userspace perspective,
> > but on the kernel side I don't think it can be implemented cleanly.
> > poll(2) is tied to the file_operations->poll() callback and
> > read/write/error events. Not to mention there isn't a way to substitue
> > the vhost worker thread function instead of scheduling out the current
> > thread while waiting for poll fd events.
> > 
> > But maybe ioctl(VHOST_WORKER_RUN) can do it:
> > 
> >  struct vhost_run_worker_dev {
> >      int vhostfd;      /* /dev/vhost-TYPE fd */
> >      unsigned nvqs;    /* number of virtqueues in vqs[] */
> >      unsigned vqs[];   /* virtqueues to process */
> >  };
> > 
> >  struct vhost_run_worker_info {
> >       struct timespec *timeout;
> >       sigset_t *sigmask;
> > 
> >       unsigned ndevices;
> >       struct vhost_run_worker_dev *devices[];
> >  };
> > 
> > In the simple case userspace sets ndevices to 1 and we just handle
> > virtqueues for the current device.
> > 
> > In the fancier shared worker thread case the userspace process has the
> > vhost fds of all the devices it is processing and passes them to
> > ioctl(VHOST_WORKER_RUN) via struct vhost_run_worker_dev elements.
> 
> Which fd will be used for this IOCTL? One of the 'vhostfd' or we should
> create a new /dev/vhost-workers (or something similar)?
> 
> Maybe the new device will be cleaner and can be reused also for other stuff
> (I'm thinking about vDPA software devices).
> 
> > 
> > From a security perspective it means the userspace thread has access to
> > all vhost devices (because it has their fds).
> > 
> > I'm not sure how the mm is supposed to work. The devices might be
> > associated with different userspace processes (guests) and therefore
> > have different virtual memory.
> 
> Maybe in this case we should do something similar to io_uring SQPOLL kthread
> where kthread_use_mm()/kthread_unuse_mm() is used to switch virtual memory
> spaces.
> 
> After writing, I saw that we already do it this in the vhost_worker() in
> drivers/vhost/vhost.c
> 
> > 
> > Just wanted to push this discussion along a little further. I'm buried
> > under emails and probably wont be very active over the next few days.
> > 
> 
> I think ioctl(VHOST_WORKER_RUN) might be the right way and also maybe the
> least difficult one.

Sending an ioctl API proposal email could help progress this discussion.

Interesting questions:
1. How to specify which virtqueues to process (Mike's use case)?
2. How to process multiple devices?

Stefan

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