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



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



 


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