[v3] Enable vhost-user to be used on BSD systems

[PATCH v3 4/4] docs: vhost-user: add subsection for non-Linux platforms

Posted by Sergio Lopez 3 years, 11 months ago

Add a section explaining how vhost-user is supported on platforms
other than Linux.

Signed-off-by: Sergio Lopez <slp@redhat.com>
---
 docs/interop/vhost-user.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
index edc3ad84a3..590a626b92 100644
--- a/docs/interop/vhost-user.rst
+++ b/docs/interop/vhost-user.rst
@@ -38,6 +38,24 @@ conventions <backend_conventions>`.
 *Master* and *slave* can be either a client (i.e. connecting) or
 server (listening) in the socket communication.
 
+Support for platforms other than Linux
+--------------------------------------
+
+While vhost-user was initially developed targeting Linux, nowadays is
+supported on any platform that provides the following features:
+
+- The ability to share a mapping injected into the guest between
+  multiple processes, so both QEMU and the vhost-user daemon servicing
+  the device can access simultaneously the memory regions containing
+  the virtqueues and the data associated with each request.
+
+- AF_UNIX sockets with SCM_RIGHTS, so QEMU can communicate with the
+  vhost-user daemon and send it file descriptors when needed.
+
+- Either eventfd or pipe/pipe2. On platforms where eventfd is not
+  available, QEMU will automatically fallback to pipe2 or, as a last
+  resort, pipe.
+
 Message Specification
 =====================
 
-- 
2.35.1

Re: [PATCH v3 4/4] docs: vhost-user: add subsection for non-Linux platforms

Posted by Stefan Hajnoczi 3 years, 11 months ago

On Thu, Mar 03, 2022 at 12:59:11PM +0100, Sergio Lopez wrote:
> Add a section explaining how vhost-user is supported on platforms
> other than Linux.
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index edc3ad84a3..590a626b92 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -38,6 +38,24 @@ conventions <backend_conventions>`.
>  *Master* and *slave* can be either a client (i.e. connecting) or
>  server (listening) in the socket communication.
>  
> +Support for platforms other than Linux
> +--------------------------------------
> +
> +While vhost-user was initially developed targeting Linux, nowadays is

s/is/it is/

> +supported on any platform that provides the following features:
> +
> +- The ability to share a mapping injected into the guest between
> +  multiple processes, so both QEMU and the vhost-user daemon servicing
> +  the device can access simultaneously the memory regions containing
> +  the virtqueues and the data associated with each request.

Please generalize this statement since there are other vhost-user
protocol features aside from guest RAM access that involve shared
memory:
1. VHOST_USER_SET_LOG_BASE
2. VHOST_USER_SET_INFLIGHT_FD

The exact requirement is:

  The vhost-user protocol relies on shared memory represented by a file
  descriptor so it can be passed over a UNIX domain socket and then
  mapped by the other process.

> +
> +- AF_UNIX sockets with SCM_RIGHTS, so QEMU can communicate with the
> +  vhost-user daemon and send it file descriptors when needed.
> +
> +- Either eventfd or pipe/pipe2. On platforms where eventfd is not
> +  available, QEMU will automatically fallback to pipe2 or, as a last

The noun is "fallback", the verb form is "fall back":
s/fallback/fall back/

It's worth mentioning that events are sent over pipe fds by writing an
8-byte value. The 8-byte value has no meaning and should not be
interpreted.

Re: [PATCH v3 4/4] docs: vhost-user: add subsection for non-Linux platforms

Posted by Michael S. Tsirkin 3 years, 11 months ago

On Thu, Mar 03, 2022 at 12:59:11PM +0100, Sergio Lopez wrote:
> Add a section explaining how vhost-user is supported on platforms
> other than Linux.
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  docs/interop/vhost-user.rst | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index edc3ad84a3..590a626b92 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -38,6 +38,24 @@ conventions <backend_conventions>`.
>  *Master* and *slave* can be either a client (i.e. connecting) or
>  server (listening) in the socket communication.
>  
> +Support for platforms other than Linux


It's not just Linux - any platform without eventfd.

So I think we should have a section explaining that whereever
spec says eventfd it can be a pipe if system does not
support creating eventfd.

> +--------------------------------------
> +
> +While vhost-user was initially developed targeting Linux, nowadays is
> +supported on any platform that provides the following features:
> +
> +- The ability to share a mapping injected into the guest between
> +  multiple processes, so both QEMU and the vhost-user daemon servicing
> +  the device can access simultaneously the memory regions containing
> +  the virtqueues and the data associated with each request.
> +
> +- AF_UNIX sockets with SCM_RIGHTS, so QEMU can communicate with the
> +  vhost-user daemon and send it file descriptors when needed.
> +
> +- Either eventfd or pipe/pipe2. On platforms where eventfd is not
> +  available, QEMU will automatically fallback to pipe2 or, as a last
> +  resort, pipe.
> +
>  Message Specification
>  =====================
>  
> -- 
> 2.35.1

Re: [PATCH v3 4/4] docs: vhost-user: add subsection for non-Linux platforms

Posted by Sergio Lopez 3 years, 11 months ago

On Fri, Mar 04, 2022 at 05:35:01AM -0500, Michael S. Tsirkin wrote:
> On Thu, Mar 03, 2022 at 12:59:11PM +0100, Sergio Lopez wrote:
> > Add a section explaining how vhost-user is supported on platforms
> > other than Linux.
> > 
> > Signed-off-by: Sergio Lopez <slp@redhat.com>
> > ---
> >  docs/interop/vhost-user.rst | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> > index edc3ad84a3..590a626b92 100644
> > --- a/docs/interop/vhost-user.rst
> > +++ b/docs/interop/vhost-user.rst
> > @@ -38,6 +38,24 @@ conventions <backend_conventions>`.
> >  *Master* and *slave* can be either a client (i.e. connecting) or
> >  server (listening) in the socket communication.
> >  
> > +Support for platforms other than Linux
> 
> 
> It's not just Linux - any platform without eventfd.
> 
> So I think we should have a section explaining that whereever
> spec says eventfd it can be a pipe if system does not
> support creating eventfd.

I'm confused. This is exactly what this subsection intends to do...

Thanks,
Sergio.

> > +--------------------------------------
> > +
> > +While vhost-user was initially developed targeting Linux, nowadays is
> > +supported on any platform that provides the following features:
> > +
> > +- The ability to share a mapping injected into the guest between
> > +  multiple processes, so both QEMU and the vhost-user daemon servicing
> > +  the device can access simultaneously the memory regions containing
> > +  the virtqueues and the data associated with each request.
> > +
> > +- AF_UNIX sockets with SCM_RIGHTS, so QEMU can communicate with the
> > +  vhost-user daemon and send it file descriptors when needed.
> > +
> > +- Either eventfd or pipe/pipe2. On platforms where eventfd is not
> > +  available, QEMU will automatically fallback to pipe2 or, as a last
> > +  resort, pipe.
> > +
> >  Message Specification
> >  =====================
> >  
> > -- 
> > 2.35.1
>