Re: [PATCH v2 1/3] xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option

From: Julien Grall <julien@xen.org>
To: Stewart Hildebrand <stewart.hildebrand@amd.com>,
	xen-devel@lists.xenproject.org
Cc: "Rahul Singh" <rahul.singh@arm.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Bertrand Marquis" <bertrand.marquis@arm.com>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Roger Pau Monné" <roger.pau@citrix.com>,
	"Oleksandr Tyshchenko" <oleksandr_tyshchenko@epam.com>,
	"Artem Mygaiev" <artem_mygaiev@epam.com>,
	"Stefano Stabellini" <stefano.stabellini@amd.com>
Subject: Re: [PATCH v2 1/3] xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option
Date: Thu, 13 Jul 2023 19:40:59 +0100	[thread overview]
Message-ID: <041323d2-9114-328f-40c6-f5fed60892a5@xen.org> (raw)
In-Reply-To: <20230707014754.51333-2-stewart.hildebrand@amd.com>

Hi Stewart,

On 07/07/2023 02:47, Stewart Hildebrand wrote:
> From: Rahul Singh <rahul.singh@arm.com>
> 
> Setting CONFIG_PCI_PASSTHROUGH=y will enable PCI passthrough on ARM, even though
> the feature is not yet complete in the current upstream codebase. The purpose of
> this is to make it easier to enable the necessary configs (HAS_PCI, HAS_VPCI) for
> testing and development of PCI passthrough on ARM.
> 
> Since PCI passthrough on ARM is still work in progress at this time, make it
> depend on EXPERT.

While preparing the patch for committing, I noticed that HAS_PASSTHROUGH 
will now allow the user to select one of the IOMMU quarantine options.

There are three of them right now:
   1. none
   2. basic (i.e. faulting)
   3. scratch page

The latter is unlikely to work on Arm because we don't setup the scratch 
page. AFAIU, for that, we would need to implement the callback 
quarantine_init().

I would expect 1 and 2 work. That said, I think 1. would behave like 2. 
because on Arm the device should not be automatically re-assigned to 
dom0. I know this is correct for platform device, but will it be valid 
for PCI as well?

Overall, before enabling HAS_PASSTHROUGH, I think there would be a bit 
of tweaking necessary to at least prevent a user to select the option 3 
(either via Kconfig or the command line). And possibly update the 
Kconfig for IOMMU_QUARANTINE to reflect the behavior on Arm.

Cheers,

-- 
Julien Grall