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

[Alex Williamson]

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

> > 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,