Re: [Qemu-devel] [RFC] libvhost-user: implement VHOST_USER_PROTOCOL_F_KICK_CALL_MSGS

[Johannes Berg]

Hi,

On Fri, 2019-09-06 at 10:22 -0400, Michael S. Tsirkin wrote:
> On Fri, Sep 06, 2019 at 03:13:50PM +0300, Johannes Berg wrote:
> > From: Johannes Berg <address@hidden>
> > 
> > Signed-off-by: Johannes Berg <address@hidden>
> 
> a bit more content here about the motivation for this?

Heh, right, definitely needed.

I was just sending it out as the corresponding patch to the spec change
RFC, where I explained more, so didn't really bother here yet. However,
I evidently forgot to CC you on that:

https://lore.kernel.org/qemu-devel/address@hidden/

I'm still trying to implement it in User-Mode Linux (UML, ARCH=um),
we've submitted patches for virtio/vhost-user support to that, but the
simulation-bound IRQ handling is a bit complicated. I need to see how it
turns out once I actually get it to work - I've gotten this extension,
SLAVE_REQ and REPLY_ACK to work now, so need to "just" integrate with
the time-travel mode I already have.

In any case, if you think that this is a stupid extension and say you
will never accept it, I'll probably just implement a slightly more
hackish way, setting vhost-user to polling mode and using out-of-band
signalling or so. This seems a bit cleaner though, and if it's properly
spec'ed and with sample code and all then it'll possibly be far more
useful to others. (**)



I think I also forgot to CC you on these two:
https://lore.kernel.org/qemu-devel/address@hidden/
https://lore.kernel.org/qemu-devel/address@hidden/

Again, sorry about that.

Btw, at least one of these files doesn't even have an entry in the
maintainers file. Don't remember if it was the spec though or the
libvhost-user stuff.


(**) For example, there's the VMSimInt paper (***) that shows a very
similar thing with QEMU, but works only with CPU emulation. With UML's
time-travel mode made to work over virtio we can do similar things
without CPU emulation. I suspect it's also possible to emulate the HPET
or so in a KVM-based system, but seems far more tricky (to me at least).

(***) 
http://www.ikr.uni-stuttgart.de/Content/Publications/Archive/We_SIMUTools_2014_40209.pdf

Thanks,
johannes