On 24/06/16 9:15 pm, "Felipe Franciosi" <fel...@nutanix.com> wrote:
>We talked to MST on IRC a while back and he brainstormed the idea of doing >this per-message. >(I even recall proposing to call this feature REPLY_ALL and he suggested >REPLY_ANY due to that.) > >I agree with doing it per message, as the protocol itself should be flexible >in that sense. >(Even if qemu today will probably want to ask for a reply in all messages.) In fact, the current implementation does exactly this. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, the current QEMU patch sets the NEED_RESPONSE flag bit for all outgoing messages — basically enforcing the vhost-user application to respond to all messages. > >On 24/06/2016, 14:59, "Qemu-devel on behalf of Marc-André Lureau" ><qemu-devel-bounces+felipe=nutanix....@nongnu.org on behalf of >marcandre.lur...@gmail.com> wrote: > >Hi > >On Fri, Jun 24, 2016 at 10:17 AM, Prerna Saxena <saxenap....@gmail.com> 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 >> >> 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 like the idea, as I encountered a similar issue in my >"vhost-user-gpu" development (which I worked around by sending a dump >GET_FEATURES.. to sync things). But I question the need to have a flag >per message. I think if the protocol feature is negociated, all >messages should have a reply. Why do you want it to be per-message? > >thanks > >-- >Marc-André Lureau > > >
