Re: [PATCH v8] xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 7/23/25 18:16, Stewart Hildebrand wrote:
> On 7/23/25 10:59, Jan Beulich wrote:
>> On 23.07.2025 16:26, Stewart Hildebrand wrote:
>>> On 7/23/25 06:18, Jan Beulich wrote:
>>>> On 13.06.2025 17:17, Stewart Hildebrand wrote:
>>>>> --- a/xen/arch/arm/Kconfig
>>>>> +++ b/xen/arch/arm/Kconfig
>>>>> @@ -8,6 +8,8 @@ config ARM_64
>>>>>   depends on !ARM_32
>>>>>   select 64BIT
>>>>>   select HAS_FAST_MULTIPLY
>>>>> + select HAS_VPCI_GUEST_SUPPORT if PCI_PASSTHROUGH
>>>>> + select HAS_PASSTHROUGH if PCI_PASSTHROUGH
>>>>
>>>> As I just learned, this change (or maybe it was the "select HAS_PCI"
>>>> further down) has exposed the quarantining Kconfig option prompt, which
>>>> (aiui) is entirely meaningless on Arm. IOW I think further adjustments
>>>> are necessary.
>>>
>>> Not entirely meaningless - the choice between "none" and "basic" still
>>> seems relevant. Just "scratch page" quarantining hasn't been implemented
>>> in any of the Arm iommu drivers.
>>
>> Is there support for "basic"? For x86, most of the involved code lives
>> in the vendor-specific drivers, and I can't find anything related in
>> Arm's. Note in particular the hook iommu_quarantine_dev_init() calls,
>> which isn't provided by any of the Arm drivers afaict.
> 
> Quoting xen/drivers/passthrough/Kconfig IOMMU_QUARANTINE_* help text:
> 
> "... basic form, all in-flight DMA will simply be forced to encounter
> IOMMU faults."
> 
> My understanding of "basic" is that after destruction of a domU with a
> PCI device assigned, the PCI device gets assigned to domIO. We have
> special casing for ( d == dom_io ) in some instances, but otherwise it
> has relatively little to do with the individual iommu drivers. I believe
> assigning to domIO should be enough for the Arm iommus to generate
> faults, since the iommu identifies the PCI device's DMA via sideband
> information (AXI stream ID).

Oh, and also note the commit messages in
63919fc4d1ca ("xen/arm: smmuv3: Add PCI devices support for SMMUv3")
and
ca8f6ffeb6e3 ("xen/arm: smmuv2: Add PCI devices support for SMMUv2")
"Implement basic quarantining."