Re: [PATCH v3 3/4] PCI: Disable ACS SV capability for the broken IDT switches

Robin Murphy
2026-02-06T14:43:43Z

On 2026-02-06 2:30 pm, Jason Gunthorpe wrote:
> On Fri, Feb 06, 2026 at 02:41:36PM +0530, Manivannan Sadhasivam wrote:
>>> It'd be worth expanding on this and what the effect of avoiding ACS SV
>>> is. Does this change which devices can be safely passed through to
>>> virtual guests? Does it give up isolation that users expect?
>>>
>>
>> IMO, ACS SV is somewhat broken on this switch. But we can still passthrough the
>> downstream devices to the guests. There won't be ACS SV apparently, but that's
>> what users will get with broken hw.
>
> I agree with this, the HW is very broken, let's have it at least work
> properly in Linux on bare metal out of the box.
>
> If someone really insists they need virtualization with narrow groups
> on this HW then they need to come with a more complete fix. Using VFIO
> is going to open up the reset flows that are problematic with the
> current solution, so it isn't like that is already working fully.
>
> Somehow I suspect nobody would use this switch for virtualization :)

And it's fine on Juno, since due to the (lack of) SMMU StreamIDs you can
only assign the entire root complex as a single iommu_group anyway :D

Cheers,
Robin.