Re: [PATCH v4 9/9] vhost_user.rst: Add MEM_READ/WRITE messages

[Albert Esteve <aesteve@redhat.com>]

On Tue, Feb 18, 2025 at 12:00 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Mon, Feb 17, 2025 at 05:40:12PM +0100, Albert Esteve wrote:
> > Add MEM_READ/WRITE request to the vhost-user
> > spec documentation.
> >
> > Signed-off-by: Albert Esteve <aesteve@redhat.com>
> > ---
> >  docs/interop/vhost-user.rst | 33 +++++++++++++++++++++++++++++++++
> >  1 file changed, 33 insertions(+)
> >
> > diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> > index 96156f1900..9f7a2c4cf7 100644
> > --- a/docs/interop/vhost-user.rst
> > +++ b/docs/interop/vhost-user.rst
> > @@ -391,6 +391,7 @@ In QEMU the vhost-user message is implemented with the following struct:
> >            VhostUserTransferDeviceState transfer_state;
> >            VhostUserMMap mmap;
> >            VhostUserShMemConfig shmem;
> > +          VhostUserMemRWMsg mem_rw;
>
> Is this struct defined anywhere in the spec?

Ah, no, right. I did not see all the other structs defined, so I
assumed some were just to be found in the code. But then it wouldn't
serve as a specification... I realized I was not looking at the right
section.

VhostUserMMap is also missing, then. I will add them in the next
iteration in their respective commits.

>
> >        };
> >    } QEMU_PACKED VhostUserMsg;
> >
> > @@ -1938,6 +1939,38 @@ is sent by the front-end.
> >    given range shall correspond to the entirety of a valid mapped region.
> >    A reply is generated indicating whether unmapping succeeded.
> >
> > +``VHOST_USER_BACKEND_MEM_READ``
> > +  :id: 11
> > +  :equivalent ioctl: N/A
> > +  :request payload: ``struct VhostUserMemRWMsg``
> > +  :reply payload: N/A
> > +
> > +  When the ``VHOST_USER_PROTOCOL_F_SHMEM`` protocol feature has been
> > +  successfully negotiated, this message can be submitted by the backends to
> > +  read a memory region that has failed to resolve a translation due to an
> > +  incomplete memory table, after another device called
> > +  ``VHOST_USER_BACKEND_SHMEM_MAP`` for the same region on a shared
> > +  descriptor file.
> > +
> > +  This mechanism works as a fallback for resolving those memory
> > +  accesses and ensure that DMA works with Shared Memory Regions.
> > +
> > +``VHOST_USER_BACKEND_MEM_WRITE``
> > +  :id: 12
> > +  :equivalent ioctl: N/A
> > +  :request payload: ``struct VhostUserMemRWMsg``
> > +  :reply payload: N/A
> > +
> > +  When the ``VHOST_USER_PROTOCOL_F_SHMEM`` protocol feature has been
> > +  successfully negotiated, this message can be submitted by the backends to
> > +  write a memory region that has failed due to resolve a translation an
> > +  incomplete memory table  after another device called
> > +  ``VHOST_USER_BACKEND_SHMEM_MAP`` for the same region on a shared
> > +  descriptor file.
> > +
> > +  This mechanism works as a fallback for resolving those memory
> > +  accesses and ensure that DMA works with Shared Memory Regions.
> > +
> >  .. _reply_ack:
> >
> >  VHOST_USER_PROTOCOL_F_REPLY_ACK
> > --
> > 2.48.1
> >