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, 20 Jul 2023 10:20:11 +0100	[thread overview]
Message-ID: <2053098f-5311-89ec-294a-aa731f2b01e1@xen.org> (raw)
In-Reply-To: <1a8d7024-f881-a3f4-c575-240fb393555f@amd.com>

Hi,

On 18/07/2023 18:35, Stewart Hildebrand wrote:
> On 7/13/23 14:40, Julien Grall wrote:
>> 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?
> 
> In a system with dom0 where the guest is created from the xl toolstack, we rely on "xl pci-assignable-add". Upon domain destruction, the device automatically gets assigned to domIO.

Ok. To clarify, does this mean any DMA will fault, the same as for 
platform device?

> However, there's nothing preventing a user from attempting to invoke "xl pci-assignable-remove", which should assign the device back to dom0, but it is not automatic.

I don't think we want to fully prevent a user to re-assign a device to 
dom0. But we at least want to avoid re-assigning the device to dom0 by 
default. After that a user can reset the device before it gets 
re-assigned to dom0.

Cheers,

-- 
Julien Grall