[PATCH RFCv1 1/3] PCI: Allow ATS to be always on for CXL.cache capable devices

[Nicolin Chen]

On Thu, Feb 19, 2026 at 10:37:37AM -0400, Jason Gunthorpe wrote:
> On Wed, Feb 18, 2026 at 02:56:35PM -0800, Nicolin Chen wrote:
> > On Tue, Feb 03, 2026 at 01:55:40PM -0400, Jason Gunthorpe wrote:
> > > On Tue, Feb 03, 2026 at 09:45:17AM -0800, Nicolin Chen wrote:
> > > > Btw, attaching to IOMMU_DOMAIN_BLOCKED/group->blocking_domain is not
> > > > allowed in general if require_direct=true. I assume this case can be
> > > > an exception since there's no point in allowing a device that has no
> > > > driver yet to access any reserved region?
> > > 
> > > If require_direct is set then we have to disable this mechanism..
> > 
> > I found a corner case, which might be another exception here?
> 
> I don't think this blocking security work needs to be part of this
> series. We just need to disable the mechanism for untrusted devices.

Oh, I thought it should be a prerequisite. I'll separate the patch
then.

> > Most of dma_configure callback functions don't use default domain
> > when driver_managed_dma is set. And this breaks MSI on pcieports.
> 
> The ARM MSI aperture need is some special case here. Those drivers
> don't use DMA at all so of course they don't have the DMA API setup,
> but they do use the MSI aperture on ARM.
> 
> Broadly here we were talking about blocked domains for unattached
> drivers, but an empty DMA domain is the same thing and still continues
> to allow the MSI vectors to work.

I see.

> So we can reframe this a little bit into more like
> 
> if the user requests IDENTITY then the IDENTITY domain is not
> installed until just before the driver binds. Up until then it is in
> the DMA domain. Meaning if userspace controls driver binding then
> unbound drivers have their DMA access blocked by an empty DMA domain.

The thing is that those driver_managed_dma callbacks don't call
iommu_device_use_default_domain(). So, the iommu core loses the
trigger to switch domain from BLOCKED/empty-DMA to DMA/IDENTITY.

The pcieports case via pci_dma_configure() is an example.

My previous approach using BUS_NOTIFY could likely work, but it
needed to adjust the timing between BUS_NOTIFY and dev->driver
setting.

Thanks
Nicolin