Re: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client responses to vhost commands.

[Prerna Saxena]

On 26/06/16 8:15 am, "Michael S. Tsirkin" <address@hidden> wrote:

>On Sat, Jun 25, 2016 at 03:13:54AM +0000, Prerna Saxena wrote:
>> 
>> 
>> 
>> 
>> 
>> On 25/06/16 4:43 am, "Michael S. Tsirkin" <address@hidden> wrote:
>> 
>> >On Fri, Jun 24, 2016 at 05:39:31PM +0000, Prerna Saxena wrote:
>> >> 
>> >> 
>> >> On 24/06/16 9:15 pm, "Felipe Franciosi" <address@hidden> 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.
>> >
>> >
>> >This seems unnecessary. Let's only do that for messages that actually
>> >need to be synchronous.
>> 
>> It would be nice to distinguish the vhost-user protocol itself from its QEMU 
>> implementation.
>> The protocol should, in theory, have provision for an implementation (such 
>> as QEMU’s vhost-user implementation) to seek response for _any_ command. 
>> However, we can choose to be selective in our QEMU implementation and just 
>> have limited commands currently send a response, such as SET_MEM_TABLE. 
>> 
>> In other words, we will still require the NEED_RESPONSE flag bit defined, 
>> but we can just set it to 1 it for SET_MEM_TABLE command in our QEMU 
>> implementation. All other vhost-user commands are sent from QEMU setting 
>> this to 0, so the application does not send an ack.
>> 
>> Michael, Does that correctly summarize what you were meaning to suggest here 
>> ?
>> 
>> Regards,
>> Prerna
>
>Exactly.

Thanks for your response. I will rework and send out a patch to that end.

Regards,
Prerna

>
>> 
>> >
>> >> >
>> >> >On 24/06/2016, 14:59, "Qemu-devel on behalf of Marc-André Lureau" 
>> >> ><address@hidden on behalf of address@hidden> wrote:
>> >> >
>> >> >Hi
>> >> >
>> >> >On Fri, Jun 24, 2016 at 10:17 AM, Prerna Saxena <address@hidden> wrote:
>> >> >> From: Prerna Saxena <address@hidden>
>> >> >>
>> >> >> 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
>> >> >
>> >> >
>> >> >