On Tue, Sep 17, 2019 at 02:26:24PM +0200, Johannes Berg wrote: > From: Johannes Berg <johannes.b...@intel.com> > > 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 <johannes.b...@intel.com> > --- > docs/interop/vhost-user.rst | 120 +++++++++++++++++++++++++++++++----- > 1 file changed, 103 insertions(+), 17 deletions(-)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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