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: <aYJLadHbJ5ynBrLH@Asurada-Nvidia>
Date: Tue, 3 Feb 2026 11:24:25 -0800
From: Nicolin Chen <nicolinc@...dia.com>
To: Robin Murphy <robin.murphy@....com>
CC: Jason Gunthorpe <jgg@...dia.com>, <dan.j.williams@...el.com>, "Tian,
 Kevin" <kevin.tian@...el.com>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, "will@...nel.org" <will@...nel.org>,
	"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 06:59:35PM +0000, Robin Murphy wrote:
> On 2026-02-03 5:55 pm, 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?
> 
> No, the point of RMRs in general is that the device can be assumed to
> already be accessing them, and that access must be preserved, regardless of
> whether an OS driver may or may not take over the device later.

I see. Thanks for the input.

> In fact RMRs
> with the "Remapping Permitted" flag are only strictly needed *until* an OS
> driver has taken control over whatever it was that firmware left them doing.

Yes. I see that doesn't set require_direct.

Nicolin