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

[Michael S. Tsirkin]

On Thu, Mar 18, 2010 at 12:40:36PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <address@hidden> wrote:
> > 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
> > ...
> 
> It makes my vmstate work more complex, because this is the only user
> that calls a vvnmstat* and uses _nothing_ of it.
> 
> rest of users are something like:
> 
> VMSTATE(...., test_function);
> 
> on the caller side.  As virtio already needs to know msix use for other
> reasons, I really expect that anything else that uses msix would need
> the same test for the same reason.
> 
> Later, Juan.


Well, ok.  If you add assumptions about callers please add a comment
before function thought,

-- 
MST