[Qemu-devel] Re: [PATCH 3/3] msi: Store the capability size in PCIDevice

[Michael S. Tsirkin]

On Tue, Nov 02, 2010 at 08:23:10AM -0600, Alex Williamson wrote:
> On Tue, 2010-11-02 at 16:07 +0200, Michael S. Tsirkin wrote:
> > On Tue, Nov 02, 2010 at 08:00:38AM -0600, Alex Williamson wrote:
> > > On Tue, 2010-11-02 at 11:25 +0200, Michael S. Tsirkin wrote:
> > > > On Mon, Nov 01, 2010 at 11:37:53PM -0600, Alex Williamson wrote:
> > > > > Avoid needing to get the MSI capability flags every time we need to
> > > > > check the capability length.  This also makes it accessible outside
> > > > > of msi.c, making it easier for users to filter config space writes
> > > > > using msi_cap and msi_cap_size.
> > > > 
> > > > I think for this last use-case, we are better off with returning a
> > > > boolean from msi_write_config which tells us whether the write is in
> > > > range. This has the advantage in that it will also work well for other
> > > > capabilities. Or second best, if that is insufficient for some reason,
> > > > export an msi_cap_size function.
> > > 
> > > Returning whether the write was in range isn't enough.  For device
> > > assignment, I need to know whether the capability was enabled or
> > > disabled.  This currently means checking the enable state before and
> > > after calling msi_write_config and doing the appropriate backend setup.
> > 
> > Sounds good. Why does this mean you need the capability size?
> >     bool was_enabled = msi_enabled(dev);
> >     msi_write_config(..)
> >     if (was_enabled != msi_enabled(dev)) {
> >     }
> 
> Because this code makes me sad...
> 
>    bool msi_was_enabled, msix_was_enabled, msi_is_enabled, msix_is_enabled;
> 
>    msi_was_enabled = msi_enabled(dev);
>    msix_was_enabled = msix_enabled(dev);
> 
>    pci_default_write_config(...
>    msi_write_config(...
>    msix_write_config(...
> 
>    msi_is_enabled = msi_enabled(dev);
>    msix_is_enabled = msix_enabled(dev);
> 
>    if (msi_was_enabled && !msi_is_enabled)
>        disable_msi(...
>    if (!msi_was_enabled && msi_is_enabled)
>        enable_msi(...
>    if (msix_was_enabled && !msix_is_enabled)
>        disable_msi(...
>    if (!msix_was_enabled && msix_is_enabled)
>        enable_msix(...
> 
> Confining msi tests to an msi related write and msix tests to an msix
> related write makes me slightly happier.  I really think we need
> callbacks though so common msi/msix code can figure out if we've made a
> transition.
> 
> Alex

This is what we have in qemu-kvm for vhost now, and the code turned out
to be terribly hard to get right.  I would rather not repeat that,
and I would love to rip out the callbacks we have now, too.
One approach would be to simply fold the handling of irqfds
into msix.c.

Having said all that, I really don't understand why does VFIO
force you to figure out that e.g. msix was enabled/disabled.
Can we not get the config write and simply call write() on VFIO?
That is an interface that makes sense to me.


> > > I think the only way I could blindly call the msi/x write config
> > > routines is if we init the capability with enable/disable callbacks.
> > > I'd be ok with an msi_cap_size function if we don't want to go that far
> > > too.  What do you prefer?  Thanks,
> 
>