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Re: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for cl

From: Prerna Saxena
Subject: Re: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client responses to vhost commands.
Date: Sat, 25 Jun 2016 03:13:54 +0000

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 
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 ?


>> >
>> >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
>> >
>> >
>> >

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