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

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Message-ID: <aYJDf7fWIxH1017J@Asurada-Nvidia>
Date: Tue, 3 Feb 2026 10:50:39 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: <dan.j.williams@...el.com>, "Tian, Kevin" <kevin.tian@...el.com>,
	"Jonathan Cameron" <jonathan.cameron@...wei.com>, "will@...nel.org"
	<will@...nel.org>, "robin.murphy@....com" <robin.murphy@....com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, "joro@...tes.org"
	<joro@...tes.org>, "praan@...gle.com" <praan@...gle.com>,
	"baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.com>,
	"miko.lenczewski@....com" <miko.lenczewski@....com>,
	"linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-cxl@...r.kernel.org"
	<linux-cxl@...r.kernel.org>
Subject: Re: [PATCH RFCv1 1/3] PCI: Allow ATS to be always on for CXL.cache
 capable devices

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'm not sure exactly what to do about this as the require_direct comes
> from the hypervisor in a CC VM and we probably don't want to give the
> hypervisor this kind of escape hatch.
> 
> Perhaps we need to lock off to failure on CC VMs if this ever
> happens..
> 
> But baremetal should just keep working how it always worked in this
> case..

OK. I will put a note in the patch, since it would literally skip
any VM case at this moment.

I just realized a corner case, as iommu_probe_device() may attach
the device to group->domain if it's set:
https://lore.kernel.org/all/9-v5-1b99ae392328+44574-iommu_err_unwind_jgg@nvidia.com/

I am not sure about the use case, but I assume we should skip the
blocking_domain as well in this case?

Then, this makes the condition be:
+	if (!dev->driver && !group->domain && !dev->iommu->require_direct) {
+               ret = __iommu_group_alloc_blocking_domain(group);
+               if (ret)
+                       goto err_remove_gdev;
+               group->domain = group->blocking_domain;
+	}

Thanks
Nicolin