On Wed, Jul 27, 2016 at 12:56:18PM +0000, Prerna Saxena wrote: > Hi Marc, > Thanks, please find my reply inline. > > > > > > On 27/07/16 4:35 pm, "Marc-André Lureau" <marcandre.lur...@gmail.com> wrote: > > >Hi > > > >On Wed, Jul 27, 2016 at 1:52 PM, Prerna Saxena <saxenap....@gmail.com> wrote: > >> From: Prerna Saxena <prerna.sax...@nutanix.com> > >> > >> This introduces the VHOST_USER_PROTOCOL_F_REPLY_ACK. > >> > >> If negotiated, client applications should send a u64 payload in > >> response to any message that contains the "need_response" bit set > >> on the message flags. Setting the payload to "zero" indicates the > >> command finished successfully. Likewise, setting it to "non-zero" > >> indicates an error. > >> > >> Currently implemented only for SET_MEM_TABLE. > >> > >> Signed-off-by: Prerna Saxena <prerna.sax...@nutanix.com> > >> --- > >> docs/specs/vhost-user.txt | 41 +++++++++++++++++++++++++++++++++++++++++ > >> hw/virtio/vhost-user.c | 32 ++++++++++++++++++++++++++++++++ > >> 2 files changed, 73 insertions(+) > >> > >> diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt > >> index 777c49c..57df586 100644 > >> --- a/docs/specs/vhost-user.txt > >> +++ b/docs/specs/vhost-user.txt > >> @@ -37,6 +37,8 @@ consists of 3 header fields and a payload: > >> * Flags: 32-bit bit field: > >> - Lower 2 bits are the version (currently 0x01) > >> - Bit 2 is the reply flag - needs to be sent on each reply from the > >> slave > >> + - Bit 3 is the need_response flag - see > >> VHOST_USER_PROTOCOL_F_REPLY_ACK for > >> + details. > > > >Why need_response and not "need reply"? > > (I’d already pointed this out earlier, but looks like I was possibly not very > clear.) > Before deciding on the right name for Bit 3, let us see the nomenclature for > Bit 2 above : "Bit 2 is the reply flag - needs to be sent on each reply from > the slave”. > So we already have a _reply_ flag in use. If the name Bit 3 as the > _need_reply_ flag, don’t you think it would be ultra-confusing ? I found it > confusing when I reviewed the documentation with this different term. > So I chose the name need_response with much deliberation — it conveys the > essence of what this flag means to achieve, but without adding to confusion.
I don't see confusion, I think I agree with Marc André. > > > >btw, I wonder if it would be worth to introduce an enum at this point > > > >> * Size - 32-bit size of the payload > >> > >> > >> @@ -126,6 +128,8 @@ the ones that do: > >> * VHOST_GET_VRING_BASE > >> * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD) > >> > >> +[ Also see the section on REPLY_ACK protocol extension. ] > >> + > >> There are several messages that the master sends with file descriptors > >> passed > >> in the ancillary data: > >> > >> @@ -254,6 +258,7 @@ Protocol features > >> #define VHOST_USER_PROTOCOL_F_MQ 0 > >> #define VHOST_USER_PROTOCOL_F_LOG_SHMFD 1 > >> #define VHOST_USER_PROTOCOL_F_RARP 2 > >> +#define VHOST_USER_PROTOCOL_F_REPLY_ACK 3 > >> > >> Message types > >> ------------- > >> @@ -464,3 +469,39 @@ Message types > >> is present in VHOST_USER_GET_PROTOCOL_FEATURES. > >> The first 6 bytes of the payload contain the mac address of the > >> guest to > >> allow the vhost user backend to construct and broadcast the fake > >> RARP. > >> + > >> +VHOST_USER_PROTOCOL_F_REPLY_ACK: > >> +------------------------------- > >> +The original vhost-user specification only demands responses for certain > > > >responses/replies > > If you feel strongly about it, will change it here. > > > > >> +commands. This differs from the vhost protocol implementation where > >> commands > >> +are sent over an ioctl() call and block until the client has completed. > >> + > >> +With this protocol extension negotiated, the sender (QEMU) can set the > >> newly > >> +introduced "need_response" [Bit 3] flag to any command. This indicates > >> that > > > >need reply, you can remove the "newly introduced" (it's not going to > >be so new after a while) > > * need_reply = no I don’t agree, for reasons cited earlier. > * remove the “newly introduced” phrase = agree, will do. > > > > >> +the client MUST respond with a Payload VhostUserMsg indicating success or > > > >I would put right here for clarity: > > > >...MUST respond with a Payload VhostUserMsg (unless the message has > >already an explicit reply body)... > > > >alternatively, I would forbid using the bit 3 on commands that have > >already an explicit reply. > > I don’t currently have any code that raises an error for such cases. > The implementation silently ignores it. > > > > >> +failure. The payload should be set to zero on success or non-zero on > >> failure. > >> +In other words, response must be in the following format : > >> + > >> +------------------------------------ > >> +| request | flags | size | payload | > >> +------------------------------------ > >> + > >> + * Request: 32-bit type of the request > >> + * Flags: 32-bit bit field: > >> + * Size: size of the payload ( see below) > >> + * Payload : a u64 integer, where a non-zero value indicates a failure. > >> + > >> +This indicates to QEMU that the requested operation has deterministically > >> +been met or not. Today, QEMU is expected to terminate the main vhost-user > >> +loop upon receiving such errors. In future, qemu could be taught to be > >> more > >> +resilient for selective requests. > >> + > >> +Note that as per the original vhost-user protocol, the following four > >> messages > >> +anyway require distinct responses from the vhost-user client process: > > > >I don't think we need to repeat this list (already redundant with the > >list in "Communication" part, and with the message specification, 2 > >times is enough imho) > > Ok, will remove it for brevity. > > > > >> + * VHOST_GET_FEATURES > >> + * VHOST_GET_PROTOCOL_FEATURES > >> + * VHOST_GET_VRING_BASE > >> + * VHOST_SET_LOG_BASE (if VHOST_USER_PROTOCOL_F_LOG_SHMFD) > >> + > >> +For these message types, the presence of VHOST_USER_PROTOCOL_F_REPLY_ACK > >> or > >> +need_response bit being set brings no behaviourial change. > > > >Reply > > Again, I disagree, for reasons cited above. > > [..snip..] removing the rest. > > > > > > > >-- > >Marc-André Lureau > > Thanks once again for the quick review. > Let me know if this makes sense, so I’ll quickly spin a cleaned-up patch > which includes the tiny documentation changes. > > Regards, > Prerna
