[Virtio-fs] [PATCH v4 3/8] vhost-user.rst: Clarify enabling/disabling vrings

[Stefan Hajnoczi]

On Wed, Oct 04, 2023 at 02:58:59PM +0200, Hanna Czenczek wrote:
> Currently, the vhost-user documentation says that rings are to be
> initialized in a disabled state when VHOST_USER_F_PROTOCOL_FEATURES is
> negotiated.  However, by the time of feature negotiation, all rings have
> already been initialized, so it is not entirely clear what this means.
> 
> At least the vhost-user-backend Rust crate's implementation interpreted
> it to mean that whenever this feature is negotiated, all rings are to
> put into a disabled state, which means that every SET_FEATURES call
> would disable all rings, effectively halting the device.  This is
> problematic because the VHOST_F_LOG_ALL feature is also set or cleared
> this way, which happens during migration.  Doing so should not halt the
> device.
> 
> Other implementations have interpreted this to mean that the device is
> to be initialized with all rings disabled, and a subsequent SET_FEATURES
> call that does not set VHOST_USER_F_PROTOCOL_FEATURES will enable all of
> them.  Here, SET_FEATURES will never disable any ring.
> 
> This interpretation does not suffer the problem of unintentionally
> halting the device whenever features are set or cleared, so it seems
> better and more reasonable.
> 
> We can clarify this in the documentation by making it explicit that the
> enabled/disabled state is tracked even while the vring is stopped.
> Every vring is initialized in a disabled state, and SET_FEATURES without
> VHOST_USER_F_PROTOCOL_FEATURES simply becomes one way to enable all
> vrings.
> 
> Signed-off-by: Hanna Czenczek <hreitz at redhat.com>
> ---
>  docs/interop/vhost-user.rst | 32 +++++++++++++++++---------------
>  1 file changed, 17 insertions(+), 15 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
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