Linux Virtualization - Sep 2019 - [PATCH] vhost: introduce mdev based hardware backend

On Thu, Sep 26, 2019 at 04:35:18AM -0400, Michael S. Tsirkin
wrote:
> On Thu, Sep 26, 2019 at 12:54:27PM +0800, Tiwei Bie wrote:
[...]
> > diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> > index 40d028eed645..5afbc2f08fa3 100644
> > --- a/include/uapi/linux/vhost.h
> > +++ b/include/uapi/linux/vhost.h
> > @@ -116,4 +116,12 @@
> >  #define VHOST_VSOCK_SET_GUEST_CID	_IOW(VHOST_VIRTIO, 0x60, __u64)
> >  #define VHOST_VSOCK_SET_RUNNING		_IOW(VHOST_VIRTIO, 0x61, int)
> >  
> > +/* VHOST_MDEV specific defines */
> > +
> > +#define VHOST_MDEV_SET_STATE	_IOW(VHOST_VIRTIO, 0x70, __u64)
> > +
> > +#define VHOST_MDEV_S_STOPPED	0
> > +#define VHOST_MDEV_S_RUNNING	1
> > +#define VHOST_MDEV_S_MAX	2
> > +
> >  #endif
> 
> So assuming we have an underlying device that behaves like virtio:
I think they are really good questions/suggestions. Thanks!
> 
> 1. Should we use SET_STATUS maybe?
I like this idea. I will give it a try.
> 2. Do we want a reset ioctl?
I think it is helpful. If we use SET_STATUS, maybe we
can use it to support the reset.
> 3. Do we want ability to enable rings individually?
I will make it possible at least in the vhost layer.
> 4. Does device need to limit max ring size?
> 5. Does device need to limit max number of queues?
I think so. It's helpful to have ioctls to report the max
ring size and max number of queues.

Thanks!
Tiwei

> 
> -- 
> MST

On Thu, Sep 26, 2019 at 09:14:39PM +0800, Tiwei Bie
wrote:
> > 4. Does device need to limit max ring size?
> > 5. Does device need to limit max number of queues?
> 
> I think so. It's helpful to have ioctls to report the max
> ring size and max number of queues.
Also, let's not repeat the vhost net mistakes, let's lock
everything to the order required by the virtio spec,
checking status bits at each step.
E.g.:
	set backend features
	set features
	detect and program vqs
	enable vqs
	enable driver

and check status at each step to force the correct order.
e.g. don't allow enabling vqs after driver ok, etc

-- 
MST

On Thu, Sep 26, 2019 at 09:26:22AM -0400, Michael S. Tsirkin
wrote:
> On Thu, Sep 26, 2019 at 09:14:39PM +0800, Tiwei Bie wrote:
> > > 4. Does device need to limit max ring size?
> > > 5. Does device need to limit max number of queues?
> > 
> > I think so. It's helpful to have ioctls to report the max
> > ring size and max number of queues.
> 
> Also, let's not repeat the vhost net mistakes, let's lock
> everything to the order required by the virtio spec,
> checking status bits at each step.
> E.g.:
> 	set backend features
> 	set features
> 	detect and program vqs
> 	enable vqs
> 	enable driver
> 
> and check status at each step to force the correct order.
> e.g. don't allow enabling vqs after driver ok, etc
Got it. Thanks a lot!

Regards,
Tiwei
> 
> -- 
> MST

On 2019/9/26 ??9:14, Tiwei Bie wrote:
> On Thu, Sep 26, 2019 at 04:35:18AM -0400, Michael S. Tsirkin wrote:
>> On Thu, Sep 26, 2019 at 12:54:27PM +0800, Tiwei Bie wrote:
> [...]
>>> diff --git a/include/uapi/linux/vhost.h
b/include/uapi/linux/vhost.h
>>> index 40d028eed645..5afbc2f08fa3 100644
>>> --- a/include/uapi/linux/vhost.h
>>> +++ b/include/uapi/linux/vhost.h
>>> @@ -116,4 +116,12 @@
>>>   #define VHOST_VSOCK_SET_GUEST_CID	_IOW(VHOST_VIRTIO, 0x60, __u64)
>>>   #define VHOST_VSOCK_SET_RUNNING		_IOW(VHOST_VIRTIO, 0x61, int)
>>>   
>>> +/* VHOST_MDEV specific defines */
>>> +
>>> +#define VHOST_MDEV_SET_STATE	_IOW(VHOST_VIRTIO, 0x70, __u64)
>>> +
>>> +#define VHOST_MDEV_S_STOPPED	0
>>> +#define VHOST_MDEV_S_RUNNING	1
>>> +#define VHOST_MDEV_S_MAX	2
>>> +
>>>   #endif
>> So assuming we have an underlying device that behaves like virtio:
> I think they are really good questions/suggestions. Thanks!
>
>> 1. Should we use SET_STATUS maybe?
> I like this idea. I will give it a try.
>
>> 2. Do we want a reset ioctl?
> I think it is helpful. If we use SET_STATUS, maybe we
> can use it to support the reset.
>
>> 3. Do we want ability to enable rings individually?
> I will make it possible at least in the vhost layer.

Note the API support e.g set_vq_ready().

>
>> 4. Does device need to limit max ring size?
>> 5. Does device need to limit max number of queues?
> I think so. It's helpful to have ioctls to report the max
> ring size and max number of queues.

An issue is the max number of queues is done through a device specific 
way, usually device configuration space. This is supported by the 
transport API, but how to expose it to userspace may need more thought.

Thanks

>
> Thanks!

On Fri, Sep 27, 2019 at 11:27:12AM +0800, Jason Wang
wrote:
> 
> On 2019/9/26 ??9:14, Tiwei Bie wrote:
> > On Thu, Sep 26, 2019 at 04:35:18AM -0400, Michael S. Tsirkin wrote:
> > > On Thu, Sep 26, 2019 at 12:54:27PM +0800, Tiwei Bie wrote:
> > [...]
> > > > diff --git a/include/uapi/linux/vhost.h
b/include/uapi/linux/vhost.h
> > > > index 40d028eed645..5afbc2f08fa3 100644
> > > > --- a/include/uapi/linux/vhost.h
> > > > +++ b/include/uapi/linux/vhost.h
> > > > @@ -116,4 +116,12 @@
> > > >   #define VHOST_VSOCK_SET_GUEST_CID	_IOW(VHOST_VIRTIO, 0x60,
__u64)
> > > >   #define VHOST_VSOCK_SET_RUNNING		_IOW(VHOST_VIRTIO, 0x61,
int)
> > > > +/* VHOST_MDEV specific defines */
> > > > +
> > > > +#define VHOST_MDEV_SET_STATE	_IOW(VHOST_VIRTIO, 0x70,
__u64)
> > > > +
> > > > +#define VHOST_MDEV_S_STOPPED	0
> > > > +#define VHOST_MDEV_S_RUNNING	1
> > > > +#define VHOST_MDEV_S_MAX	2
> > > > +
> > > >   #endif
> > > So assuming we have an underlying device that behaves like
virtio:
> > I think they are really good questions/suggestions. Thanks!
> > 
> > > 1. Should we use SET_STATUS maybe?
> > I like this idea. I will give it a try.
> > 
> > > 2. Do we want a reset ioctl?
> > I think it is helpful. If we use SET_STATUS, maybe we
> > can use it to support the reset.
> > 
> > > 3. Do we want ability to enable rings individually?
> > I will make it possible at least in the vhost layer.
> 
> 
> Note the API support e.g set_vq_ready().
virtio spec calls this "enabled" so let's stick to that.
> 
> > 
> > > 4. Does device need to limit max ring size?
> > > 5. Does device need to limit max number of queues?
> > I think so. It's helpful to have ioctls to report the max
> > ring size and max number of queues.
> 
> 
> An issue is the max number of queues is done through a device specific way,
> usually device configuration space. This is supported by the transport API,
> but how to expose it to userspace may need more thought.
> 
> Thanks
an ioctl for device config?  But for v1 I'd be quite happy to just have
a minimal working device with 2 queues.
> 
> > 
> > Thanks!