linux-kernel - Re: [RFC 7/7] vhost: Add new UAPI to support changing Kthread mode

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Message-ID: <CACLfguVSqfrjTjYGkP6Fis=pONcf5x7u-8qUb=-Z+o-fLWENcg@mail.gmail.com>
Date: Wed, 28 Aug 2024 17:46:21 +0800
From: Cindy Lu <lulu@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: mst@...hat.com, linux-kernel@...r.kernel.org, 
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC 7/7] vhost: Add new UAPI to support changing Kthread mode

On Tue, 27 Aug 2024 at 10:32, Jason Wang <jasowang@...hat.com> wrote:
>
> On Mon, Aug 19, 2024 at 5:29 PM Cindy Lu <lulu@...hat.com> wrote:
> >
> > Add a new UAPI to support setting the vhost device to
> > use kthread mode. The user space application needs to use
> > VHOST_SET_USE_KTHREAD to set the mode. This setting must
> > be set before VHOST_SET_OWNER is set.
> >
> > Signed-off-by: Cindy Lu <lulu@...hat.com>
> > ---
> >  drivers/vhost/vhost.c      | 11 ++++++++++-
> >  include/uapi/linux/vhost.h |  2 ++
> >  2 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 0a7b2999100f..d6b71bddc272 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2340,14 +2340,23 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp)
> >  {
> >         struct eventfd_ctx *ctx;
> >         u64 p;
> > -       long r;
> > +       long r = 0;
> >         int i, fd;
> > +       bool kthread;
> >
> >         /* If you are not the owner, you can become one */
> >         if (ioctl == VHOST_SET_OWNER) {
> >                 r = vhost_dev_set_owner(d);
> >                 goto done;
> >         }
> > +       if (ioctl == VHOST_SET_USE_KTHREAD) {
> > +               if (copy_from_user(&kthread, argp, sizeof(kthread))) {
> > +                       r = -EFAULT;
> > +                       goto done;
> > +               }
> > +               use_kthread = kthread;
> > +               goto done;
> > +       }
> >
> >         /* You must be the owner to do anything else */
> >         r = vhost_dev_check_owner(d);
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index b95dd84eef2d..386fe735da63 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -235,4 +235,6 @@
> >   */
> >  #define VHOST_VDPA_GET_VRING_SIZE      _IOWR(VHOST_VIRTIO, 0x82,       \
> >                                               struct vhost_vring_state)
> > +
>
> Unnecessary changes.
>
> > +#define VHOST_SET_USE_KTHREAD _IOW(VHOST_VIRTIO, 0x83, bool)
>
> So I think we need to do something the opposite. New flag for new
> behaviour instead of new flag for the old one ...
>
> By using this we can unbreak the old applications.
>
> Btw, I think this needs to come before/along with the introduction of
> the module parameter that enforce old behavior.
>
I think the old behavior your mentioned here is use Kthread?
So If I understand correctly, the NEW uapi is something like
VHOST_SET_ENFORCE_TASK

The module parameter can also be changed to.
bool enforce_kthread = true;
module_param(enforce_true, bool, 0444);

thanks
Cindy
> Thanks
>
>
> >  #endif
>
> > --
> > 2.45.0
> >
>