Re: [Qemu-devel] [RFC][PATCH 23/45] qemu-kvm: Rework MSI-X mask notifier to generic MSI config notifiers

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 17, 2011 at 09:08:58PM +0200, Jan Kiszka wrote:
> On 2011-10-17 14:39, Michael S. Tsirkin wrote:
> > On Mon, Oct 17, 2011 at 01:45:04PM +0200, Jan Kiszka wrote:
> >> On 2011-10-17 13:40, Michael S. Tsirkin wrote:
> >>> On Mon, Oct 17, 2011 at 11:27:57AM +0200, Jan Kiszka wrote:
> >>>> MSI config notifiers are supposed to be triggered on every relevant
> >>>> configuration change of MSI vectors or if MSI is enabled/disabled.
> >>>>
> >>>> Two notifiers are established, one for vector changes and one for general
> >>>> enabling. The former notifier additionally passes the currently active
> >>>> MSI message.
> >>>> This will allow to update potential in-kernel IRQ routes on
> >>>> changes. The latter notifier is optional and will only be used by a
> >>>> subset of clients.
> >>>>
> >>>> These notifiers are currently only available for MSI-X but will be
> >>>> extended to legacy MSI as well.
> >>>>
> >>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
> >>>
> >>> Passing message, always, does not seem to make sense: message is only
> >>> valid if it is unmasked.
> >>
> >> If we go from unmasked to masked, the consumer could just ignore the
> >> message.
> > 
> > Why don't we let the consumer get the message if it needs it?
> 
> Because most consumer will need and because I want to keep the API simple.

The API seems to get more complex, as you are passing
in fields which are only valid sometimes.

> > 
> >>> Further, IIRC the spec requires any changes to be done while
> >>> message is masked. So mask notifier makes more sense to me:
> >>> it does the same thing using one notifier that you do
> >>> using two notifiers.
> >>
> >> That's in fact a possible optimization (only invoke the callback on mask
> >> transitions).
> > 
> > Further, it is one that is already implemented.
> > So I would prefer not to add work by removing it :)
> 
> Generalization to cover MSI requires some changes. Unneeded behavioral
> changes back and forth should and will of course be avoided. I will
> rework this.
> 
> > 
> >> Not sure if that applies to MSI as well, probably not.
> > 
> > Probably not. However, if per vector masking is
> > supported, and while vector is masked, the address/
> > data values might not make any sense.
> > 
> > So I think even msi users needs to know about masked state.
> 
> Yes, and they get this information via the config notifier.
> 
> > 
> >> To
> >> have common types, I would prefer to stay with vector config notifiers
> >> as name then.
> >>
> >> Jan
> > 
> > So we pass in nonsense values and ask all users to know about MSIX rules.
> > Ugh.
> > 
> > I do realize msi might change the vector without masking.
> > We can either artificially call mask before value change
> > and unmask after, or use 3 notifiers: mask,unmask,config.
> > Add a comment that config is invoked when configuration
> > for an unmasked vector is changed, and that
> > it can only happen for msi, not msix.
> 
> I see no need in complicating the API like this. MSI-X still needs the
> config information on unmask, so let's just consistently pass it via the
> unified config notifier instead of forcing the consumers to create yet
> two more handlers. I really do not see the benefit for the consumer.
> 
> Jan
> 

The benefit is a clearer API, where all parameters you get are valid,
so you do not need to go read the spec to see what is OK to use.
Generally, encoding events in flags is more error
prone than using different notifiers for different events.

E.g. _unmask and _mask make
it obvious that they are called on mask and on unmask
respectively.
OTOH _config_change(bool mask) is unclear: is 'mask' the new
state or the old state?

It might be just my taste, but I usually prefer multiple
functions doing one thing each rather than a single
function doing multiple things. It shouldn't be too hard ...

-- 
MST