Re: [Qemu-devel] [PATCH 1/2] docs: vhost-user: add in-band kick/call messages

[Stefan Hajnoczi]

On Tue, Sep 17, 2019 at 02:26:24PM +0200, Johannes Berg wrote:
> From: Johannes Berg <address@hidden>
> 
> For good reason, vhost-user is currently built asynchronously, that
> way better performance can be obtained. However, for certain use
> cases such as simulation, this is problematic.
> 
> Consider an event-based simulation in which both the device and CPU
> have scheduled according to a simulation "calendar". Now, consider
> the CPU sending I/O to the device, over a vring in the vhost-user
> protocol. In this case, the CPU must wait for the vring interrupt
> to have been processed by the device, so that the device is able to
> put an entry onto the simulation calendar to obtain time to handle
> the interrupt. Note that this doesn't mean the I/O is actually done
> at this time, it just means that the handling of it is scheduled
> before the CPU can continue running.
> 
> This cannot be done with the asynchronous eventfd based vring kick
> and call design.
> 
> Extend the protocol slightly, so that a message can be used for kick
> and call instead, if VHOST_USER_PROTOCOL_F_INBAND_NOTIFICATIONS is
> negotiated. This in itself doesn't guarantee synchronisation, but both
> sides can also negotiate VHOST_USER_PROTOCOL_F_REPLY_ACK and thus get
> a reply to this message by setting the need_reply flag, and ensure
> synchronisation this way.
> 
> To really use it in both directions, VHOST_USER_PROTOCOL_F_SLAVE_REQ
> is also needed.
> 
> Since it is used for simulation purposes and too many messages on
> the socket can lock up the virtual machine, document that this should
> only be used together with the mentioned features.
> 
> Signed-off-by: Johannes Berg <address@hidden>
> ---
>  docs/interop/vhost-user.rst | 120 +++++++++++++++++++++++++++++++-----
>  1 file changed, 103 insertions(+), 17 deletions(-)

Reviewed-by: Stefan Hajnoczi <address@hidden>

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