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

From: Prerna
Subject: Re: [Qemu-devel] [PATCH 0/1] vhost-user: Add a protocol extension for client responses to vhost commands.
Date: Fri, 24 Jun 2016 14:00:54 +0530

CC'ing MarcAndre & Michael S Tsirkin.

On Fri, Jun 24, 2016 at 1:47 PM, Prerna Saxena <address@hidden>

> 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.
> 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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