On Fri, Jun 24, 2016 at 01:17:07AM -0700, Prerna Saxena wrote: > From: Prerna Saxena <prerna.sax...@nutanix.com> > > The current vhost-user protocol requires the client to send responses to only > few commands. For the remaining commands, it is impossible for QEMU to know > the status of the requested operation -- ie, did it succeed at all, and if > so, at what time. > > This is inconvenient, and can also lead to races. As an example: > > (1) qemu sends a SET_MEM_TABLE to the backend (eg, a vhost-user net > application) and SET_MEM_TABLE doesn't require a reply according to the spec. > (2) qemu commits the memory to the guest. > (3) guest issues an I/O operation over a new memory region which was > configured on (1) > (4) The application hasn't yet remapped the memory, but it sees the I/O > request. > (5) The application cannot satisfy the request because it doesn't know about > those GPAs
OK, but there's a simple way to fix the race: submit another request e.g. GET_FEATURES and wait for a response. > > Note that the kernel implementation does not suffer from this limitation > since messages are sent via an ioctl(). The ioctl() blocks until the backend > (eg. vhost-net) completes the command and returns (with an error code). > > Changing the behaviour of current vhost-user commands would break existing > applications. This patch introduces a protocol extension, > VHOST_USER_PROTOCOL_F_REPLY_ACK. This feature, if negotiated, allows QEMU to > annotate messages to the application that it seeks a response for. The > application must then respond to qemu by providing a status about the > requested operation. I would like this structured like this: 1. fix races by sending GET_FEATURES and waiting for it 2. add VHOST_USER_PROTOCOL_F_REPLY_ACK > > Prerna Saxena (1): > vhost-user : Introduce a new feature, VHOST_USER_PROTOCOL_F_REPLY_ACK > This feature, if negotiated, forces the remote vhost-user > process to send a u64 reply containin status code for each > requested operation. > Status codes are '0' for success, and non-zero for error. > > docs/specs/vhost-user.txt | 36 +++++++++++ > hw/virtio/vhost-user.c | 153 > +++++++++++++++++++++++++++++++++++++++++++++- > 2 files changed, 186 insertions(+), 3 deletions(-) > > -- > 1.8.1.2 >
