The vhost-user spec has been updated to clarify enabling and disabling vrings. The current spec states that upon receiving a VHOST_USER_SET_FEATURES message from the front-end without VHOST_USER_F_PROTOCOL_FEATURES set, the back-end must enable all rings immediately. Also, while processing the rings (whether they are enabled or not), the back-end must support changing some configuration aspects on the fly.
However, the vhost-user-backend implementation will disable all vring whenever this feature is negotiated, which means that every VHOST_USER_SET_FEATURES message could unintentionally halt the device.
This is an incorrect interpretation of the spec, and is problematic because the VHOST_F_LOG_ALL feature is also set or cleared by VHOST_USER_SET_FEATURES, which happens during migration. And doing so should not disable any vring nor halt the device.
This code is part of the effort to support live migration in virtiofsd and is currently being used in a version of virtiofsd that supports migration: https://gitlab.com/virtiofsd-live-migration
Summary of the PR
The vhost-user spec has been updated to clarify enabling and disabling vrings. The current spec states that upon receiving a
VHOST_USER_SET_FEATURESmessage from the front-end withoutVHOST_USER_F_PROTOCOL_FEATURESset, the back-end must enable all rings immediately. Also, while processing the rings (whether they are enabled or not), the back-end must support changing some configuration aspects on the fly.However, the vhost-user-backend implementation will disable all vring whenever this feature is negotiated, which means that every
VHOST_USER_SET_FEATURESmessage could unintentionally halt the device.This is an incorrect interpretation of the spec, and is problematic because the
VHOST_F_LOG_ALLfeature is also set or cleared byVHOST_USER_SET_FEATURES, which happens during migration. And doing so should not disable any vring nor halt the device.This code is part of the effort to support live migration in virtiofsd and is currently being used in a version of virtiofsd that supports migration: https://gitlab.com/virtiofsd-live-migration