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

[Jason Gunthorpe]

On Fri, Feb 06, 2026 at 09:05:05AM -0600, Bjorn Helgaas wrote:
> On Fri, Feb 06, 2026 at 10:52:54AM -0400, Jason Gunthorpe wrote:
> > On Fri, Feb 06, 2026 at 08:46:51AM -0600, Bjorn Helgaas wrote:
> > 
> > > IIUC the current situation is that for these IDT switches, ACS SV is
> > > enabled when downstream devices are passed through to guests, but
> > > after these patches, it will no longer be enabled.
> > 
> > ACS SV is enabled at boot time if an IOMMU driver is present
> > regardless if guests or virtualization is in use.
> > 
> > Linux doesn't change ACS flags dynamically.
> 
> Right, it's just that this series effectively un-advertises ACS SV for
> the IDE switches so it will never be enabled for them

Yes

> > iommu_groups will correctly describe the system limitations with the
> > ACS quirk path and so all of the above concerns are taken care
> > of. Robin is saying the Juno SMMU forces a large iommu_group covering
> > the switch anyhow today, so at least that platform is not affected.
> 
> I guess REQ_ACS_FLAGS is what iommu_groups looks for?  I looked for
> such a thing earlier but must have missed it.  Thanks!

Yes

Jason