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From: Jan Beulich <jbeulich@suse.com>
To: Julien Grall <julien@xen.org>, Rahul Singh <rahul.singh@arm.com>
Cc: xen-devel@lists.xenproject.org, bertrand.marquis@arm.com,
	"Paul Durrant" <paul@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Ian Jackson" <iwj@xenproject.org>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Wei Liu" <wl@xen.org>, "Daniel De Graaf" <dgdegra@tycho.nsa.gov>,
	"Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI
Date: Thu, 8 Apr 2021 08:00:27 +0200	[thread overview]
Message-ID: <0071311d-6da5-636b-b823-32b5570d8f49@suse.com> (raw)
In-Reply-To: <4ab1e7ed-cdf6-1b06-4f55-bbb981cade91@xen.org>

On 07.04.2021 20:06, Julien Grall wrote:
> 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?

Minor remark: In this name I'd be inclined to suggest to omit NEED.

Jan


  reply	other threads:[~2021-04-08  6:00 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-06 11:39 [PATCH 0/2] xen/pci: Make PCI passthrough code non-x86 specific Rahul Singh
2021-04-06 11:39 ` [PATCH 1/2] xen/pci: Move PCI ATS code to common directory Rahul Singh
2021-04-06 15:16   ` Jan Beulich
2021-04-06 11:39 ` [PATCH 2/2] xen/pci: Gate all MSI code in common code with CONFIG_HAS_PCI_MSI Rahul Singh
2021-04-06 14:13   ` Roger Pau Monné
2021-04-06 14:30     ` Julien Grall
2021-04-06 14:59       ` Roger Pau Monné
2021-04-06 15:09         ` Julien Grall
2021-04-06 15:58           ` Roger Pau Monné
2021-04-07 17:52             ` Rahul Singh
2021-04-06 15:25       ` Jan Beulich
2021-04-07 18:06         ` Julien Grall
2021-04-08  6:00           ` Jan Beulich [this message]
2021-04-08  8:45             ` Rahul Singh

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