Re: [Qemu-devel] Re: [PATCH] vhost: fix features ack

[Blue Swirl <blauwirbel@gmail.com>]

On 3/31/10, Anthony Liguori <anthony@codemonkey.ws> wrote:
> On 03/31/2010 02:07 PM, Michael S. Tsirkin wrote:
>
> > On Wed, Mar 31, 2010 at 03:38:05PM -0300, Luiz Capitulino wrote:
> >
> >
> > > On Wed, 31 Mar 2010 13:26:23 -0500
> > > Anthony Liguori<anthony@codemonkey.ws>  wrote:
> > >
> > >
> > >
> > > > On 03/31/2010 01:20 PM, Michael S. Tsirkin wrote:
> > > >
> > > >
> > > > > From: David L Stevens<dlstevens@us.ibm.com>
> > > > >
> > > > > vhost driver in qemu didn't ack features, and this happens
> > > > > to work because we don't really require any features. However,
> > > > > it's better not to rely on this. This patch passes features to
> > > > > vhost as guest acks them.
> > > > >
> > > > > Signed-off-by: David L Stevens<dlstevens@us.ibm.com>
> > > > > Signed-off-by: Michael S. Tsirkin<mst@redhat.com>
> > > > > ---
> > > > >
> > > > > Anthony, here's a fixup patch to address an issue in vhost
> > > > > patches. Incidentially, what's the status of the vhost patchset?
> > > > >
> > > > >
> > > > >
> > > >
> http://repo.or.cz/w/qemu/aliguori-queue.git
> vhost
> > > >
> > > > Is what I'm currently testing.  With vhost disabled,  the following
> seg
> > > > faults:
> > > >
> > > > qemu-system-x86_64 -hda ~/images/linux.img -net tap -net
> > > > nic,model=virtio -enable-kvm
> > > >
> > > > But not when using TCG.  I'm not sure that it's your patches at fault
> > > > and I'm attempting to bisect now to figure that out.
> > > >
> > > >
> > >   Probably this is the same segfault I'm getting right
> now in master,
> > > bisect says it's:
> > >
> > > """
> > > commit ad96090a01d848df67d70c5259ed8aa321fa8716
> > > Author: Blue Swirl<blauwirbel@gmail.com>
> > > Date:   Mon Mar 29 19:23:52 2010 +0000
> > >
> > >     Refactor target specific handling, compile vl.c only once
> > > """
> > >
> > >
> >  Why are the compile once patches helpful? They seem to
> introduce
> > churn and bugs, they actively make it harder to extend qemu as you can't
> use
> > target-specific code in code that is compiled once, they might have
> > performance penalty - and what do we gain? Any given user is unlikely to
> > need to build on more than one target, distros have enough computing
> > power to build in parallel.
> >
> > Maybe it makes sense to revert the compile once patches, and discuss
> > these issues before re-commit?
> >
> >
>
>  Compiling objects once is certainly useful.  Long term, I think most of us
> want to see a single qemu executable that works for all architectures and
> compiling once is an important step in that direction.
>
>  With respect to regressions, it might make sense to slow down these
> refactorings a bit and increase the amount of regression testing that is
> happening during them.

I think there are only a few useful refactorings left. MIPS was
interesting because of fourfold savings, likewise triple savings with
PPC. Refactoring i386/x86_64 devices may be worthwhile, the rest not.