Re: [RFC 02/10] vhost: add 3 commands for vhost-vdpa

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Cornelia Huck <cohuck@redhat.com>,
	qemu-devel <qemu-devel@nongnu.org>, Yechuan <yechuan@huawei.com>,
	"Gonglei \(Arei\)" <arei.gonglei@huawei.com>,
	Huangzhichao <huangzhichao@huawei.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	pbonzini <pbonzini@redhat.com>,
	"Longpeng\(Mike\)" <longpeng2@huawei.com>,
	Stefano Garzarella <sgarzare@redhat.com>
Subject: Re: [RFC 02/10] vhost: add 3 commands for vhost-vdpa
Date: Wed, 5 Jan 2022 17:21:02 +0800	[thread overview]
Message-ID: <CACGkMEuJryvBzAZOaqP4QV=aj0Rf95C_kWpg9dh4tsQXdbEUaw@mail.gmail.com> (raw)
In-Reply-To: <20220105030025-mutt-send-email-mst@kernel.org>

On Wed, Jan 5, 2022 at 5:12 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Jan 05, 2022 at 03:54:13PM +0800, Jason Wang wrote:
> > On Wed, Jan 5, 2022 at 3:02 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Wed, Jan 05, 2022 at 12:35:53PM +0800, Jason Wang wrote:
> > > > On Wed, Jan 5, 2022 at 8:59 AM Longpeng(Mike) <longpeng2@huawei.com> wrote:
> > > > >
> > > > > From: Longpeng <longpeng2@huawei.com>
> > > > >
> > > > > To support generic vdpa deivce, we need add the following ioctls:
> > > > > - GET_VECTORS_NUM: the count of vectors that supported
> > > >
> > > > Does this mean MSI vectors? If yes, it looks like a layer violation:
> > > > vhost is transport independent.
> > >
> > > Well *guest* needs to know how many vectors device supports.
> > > I don't think there's a way around that. Do you?
> >
> > We have VHOST_SET_VRING/CONFIG_CALL which is per vq. I think we can
> > simply assume #vqs + 1?
>
> Some hardware may be more limited. E.g. there could be a bunch of vqs
> which don't need a dedicated interrupt. Or device could support a single
> interrupt shared between VQs.

Right, but in the worst case, we may just waste some eventfds?

Or if we want to be optimized in the case you mentioned above, we need
to know the association among vectors and vqs which requires more
extensions.

1) API to know which vectors does a dedicated VQ belong
2) the kick eventfd is set based on the vectors but not vq

And such mappings are not static, e.g IFCVF requesting MSI-X vectors
during DRIVER_OK.

Thanks

>
>
> > > Otherwise guests will at best be suboptimal.
> > >
> > > >  And it reveals device implementation
> > > > details which block (cross vendor) migration.
> > > >
> > > > Thanks
> > >
> > > Not necessarily, userspace can hide this from guest if it
> > > wants to, just validate.
> >
> > If we can hide it at vhost/uAPI level, it would be even better?
> >
> > Thanks
> >
> > >
> > >
> > > > > - GET_CONFIG_SIZE: the size of the virtio config space
> > > > > - GET_VQS_NUM: the count of virtqueues that exported
> > > > >
> > > > > Signed-off-by: Longpeng <longpeng2@huawei.com>
> > > > > ---
> > > > >  linux-headers/linux/vhost.h | 10 ++++++++++
> > > > >  1 file changed, 10 insertions(+)
> > > > >
> > > > > diff --git a/linux-headers/linux/vhost.h b/linux-headers/linux/vhost.h
> > > > > index c998860d7b..c5edd75d15 100644
> > > > > --- a/linux-headers/linux/vhost.h
> > > > > +++ b/linux-headers/linux/vhost.h
> > > > > @@ -150,4 +150,14 @@
> > > > >  /* Get the valid iova range */
> > > > >  #define VHOST_VDPA_GET_IOVA_RANGE      _IOR(VHOST_VIRTIO, 0x78, \
> > > > >                                              struct vhost_vdpa_iova_range)
> > > > > +
> > > > > +/* Get the number of vectors */
> > > > > +#define VHOST_VDPA_GET_VECTORS_NUM     _IOR(VHOST_VIRTIO, 0x79, int)
> > > > > +
> > > > > +/* Get the virtio config size */
> > > > > +#define VHOST_VDPA_GET_CONFIG_SIZE     _IOR(VHOST_VIRTIO, 0x80, int)
> > > > > +
> > > > > +/* Get the number of virtqueues */
> > > > > +#define VHOST_VDPA_GET_VQS_NUM         _IOR(VHOST_VIRTIO, 0x81, int)
> > > > > +
> > > > >  #endif
> > > > > --
> > > > > 2.23.0
> > > > >
> > >
>