Re: [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 06/04/2021 16:25, Jan Beulich wrote:
> On 06.04.2021 16:30, Julien Grall wrote:
> > Hi Roger,
> >
> > On 06/04/2021 15:13, Roger Pau Monné wrote:
> > > On Tue, Apr 06, 2021 at 12:39:11PM +0100, Rahul Singh wrote:
> > > > MSI support is not implemented for ARM architecture but it is enabled
> > > > for x86 architecture and referenced in common passthrough/pci.c code.
> > > >
> > > > Therefore introducing the new flag to gate the MSI code for ARM in
> > > > common code to avoid compilation error when HAS_PCI is enabled for ARM.
> > > Is such option really interesting long term?
> > >
> > > IIRC PCI Express mandates MSI support, at which point I don't see much
> > > value in being able to compile out the MSI support.
> > I am pretty sure there are board out with PCI support but no MSI
> > support. Anyway, even if the spec may mandate it...
> >
> > > So while maybe helpful for Arm PCI efforts ATM, I'm not sure it
> > > warrants a Kconfig option, I would rather see Arm introduce dummy
> > > helpers for the missing functionality, even if unimplemented at the
> > > moment.
> > ... from my understanding, most of (if not all) the MSI code is not very
> > useful on Arm when using the GICv3 ITS.
> >
> > The GICv3 ITS will do the isolation for you and therefore we should not
> > need to keep track of the state at the vPCI level.
> But that's then not "has PCI MSI" but "need to intercept PCI MSI
> accesses", i.e. I don't think the Kconfig option is correctly
> named. If a device with MSI support is used, you can't make that
> MSI support go away, after all.
That's actually a good point. Rahul, do you think the config can be 
renamed to something like CONFIG_PCI_MSI_NEED_INTERCEPT?
Cheers,

--
Julien Grall