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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: Fri, 24 Jun 2016 17:39:31 +0000

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.

>On 24/06/2016, 14:59, "Qemu-devel on behalf of Marc-André Lureau" 
><address@hidden on behalf of address@hidden> wrote:
>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?
>Marc-André Lureau

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