[Qemu-devel] Re: [PATCH 6/9] virtio-pci: Remove duplicate test

[Michael S. Tsirkin]

On Thu, Mar 18, 2010 at 09:59:03AM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <address@hidden> wrote:
> > On Thu, Mar 18, 2010 at 09:26:10AM +0100, Juan Quintela wrote:
> >> "Michael S. Tsirkin" <address@hidden> wrote:
> >> > On Tue, Mar 16, 2010 at 07:51:22PM +0100, Juan Quintela wrote:
> >> >> We already do the test for msix on the caller, just use that test
> >> >> 
> >> >> Signed-off-by: Juan Quintela <address@hidden>
> >> >
> >> > NAK
> >> >
> >> > I think we are better off not making assumptions
> >> > about caller behaviour in msix.c, virtio
> >> > will not be the only user forever.
> >> 
> >> That makes migration testing more difficult.  Basically we are testing
> >> if we are using msix in two places.  Obvious thing is:
> >> - we don't test in msix_save() if msix is used.
> >> - we don't test it in virtio_pci_save_config()
> >> 
> >> I don't care if it is one way or another, but requiring to check it in
> >> the caller and the callee is a bit too much for me.
> >> 
> >> Later, Juan.
> >
> > msix does not require the check in the caller, by design it is
> > safe to call msix_save when msix is not present.
> 
> look at it, it requires to test msix support for other things, which
> amount to the same thing :(
> 
> Later, Juan.

Yes, but it does not have to be the only user. We'll have at least pci
assignment use it, at some point.  IOW, msix tries to present an API
that is safe to use, and checks capability so we do not crash if it's
not there.  virtio happens to need the same test for its own reasons
(vmsave format backwards compatibility).

We could optimize the test if we cared about speed here, but we don't
...

-- 
MST