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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client responses to vhost commands.
Date: Sat, 25 Jun 2016 01:43:33 +0300

On Fri, Jun 24, 2016 at 01:17:07AM -0700, Prerna Saxena 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

OK, but there's a simple way to fix the race: submit
another request e.g. GET_FEATURES and wait for a response.

> 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 would like this structured like this:

1. fix races by sending GET_FEATURES and waiting for it

> Prerna Saxena (1):
>   vhost-user : Introduce a new feature, VHOST_USER_PROTOCOL_F_REPLY_ACK 
>                This feature, if negotiated, forces the remote vhost-user
>                process to send a u64 reply containin status code for each
>                requested operation.          
>                Status codes are '0' for success, and non-zero for error.
>  docs/specs/vhost-user.txt |  36 +++++++++++
>  hw/virtio/vhost-user.c    | 153 
> +++++++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 186 insertions(+), 3 deletions(-)
> -- 

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