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 <hre...@redhat.com> > --- > docs/interop/vhost-user.rst | 32 +++++++++++++++++--------------- > 1 file changed, 17 insertions(+), 15 deletions(-)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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