Re: [Qemu-devel] [PATCH v3 for-4.0 2/7] vhost-user: Support providing shared memory to backend

[Yongji Xie]

On Fri, 4 Jan 2019 at 01:02, Michael S. Tsirkin <address@hidden> wrote:
>
> On Thu, Jan 03, 2019 at 06:18:14PM +0800, address@hidden wrote:
> > From: Xie Yongji <address@hidden>
> >
> > This patch introduces two new messages VHOST_USER_GET_SHM_SIZE
> > and VHOST_USER_SET_SHM_FD to support providing shared
> > memory to backend.
>
> So this seems a bit vague. Since we are going to use it
> for tracking in-flight I/O I would prefer it that we
> actually call it that.
>
>

So how about VHOST_USER_GET_INFLIGHT_SIZE and VHOST_USER_SET_INFLIHGT_FD?

> >
> > Firstly, qemu uses VHOST_USER_GET_SHM_SIZE to get the
> > required size of shared memory from backend. Then, qemu
> > allocates memory and sends them
>
> s/them/it/ ?
>

Will fix it in v4.

> > back to backend through
> > VHOST_USER_SET_SHM_FD.
> >
> > Note that the shared memory should be used to record
> > inflight I/O by backend. Qemu will clear it when vm reset.
> >
> > Signed-off-by: Xie Yongji <address@hidden>
> > Signed-off-by: Chai Wen <address@hidden>
> > Signed-off-by: Zhang Yu <address@hidden>
> > ---
> >  docs/interop/vhost-user.txt       |  41 +++++++++++
> >  hw/virtio/vhost-user.c            |  86 ++++++++++++++++++++++
> >  hw/virtio/vhost.c                 | 117 ++++++++++++++++++++++++++++++
> >  include/hw/virtio/vhost-backend.h |   9 +++
> >  include/hw/virtio/vhost.h         |  19 +++++
> >  5 files changed, 272 insertions(+)
> >
> > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > index c2194711d9..5ee9c28ab0 100644
> > --- a/docs/interop/vhost-user.txt
> > +++ b/docs/interop/vhost-user.txt
> > @@ -142,6 +142,19 @@ Depending on the request type, payload can be:
> >     Offset: a 64-bit offset of this area from the start of the
> >         supplied file descriptor
> >
> > + * Shm description
> > +   -----------------------------------
> > +   | mmap_size | mmap_offset | dev_size | vq_size | align | version |
> > +   -----------------------------------
> > +
> > +   Mmap_size: a 64-bit size of the shared memory
> > +   Mmap_offset: a 64-bit offset of the shared memory from the start
> > +                of the supplied file descriptor
> > +   Dev_size: a 32-bit size of device region in shared memory
> > +   Vq_size: a 32-bit size of each virtqueue region in shared memory
> > +   Align: a 32-bit align of each region in shared memory
> > +   Version: a 32-bit version of this shared memory
> > +
>
> This is an informal description so please avoid _ in field
> names, just put a space in there. See e.g. log description.
>
>
Got it!

> >  In QEMU the vhost-user message is implemented with the following struct:
> >
> >  typedef struct VhostUserMsg {
>
>
> For things to work, in-flight format must not change when
> backend reconnects.
>

I'm not sure whether there will be some cases that we want to add some fields to
the inflight area without stopping vm.

> To encourage consistency, how about including a recommended format for
> this buffer in this document?
>
>

Sure. Will add it in v4.

Thanks,
Yongji