linux-kernel - RE: [PATCH v5 5/5] pci: Suspend iommu function prior to resetting a device

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Message-ID: <BN9PR11MB527640AE172858646199B1888CD6A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Tue, 18 Nov 2025 07:53:27 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
CC: "joro@...tes.org" <joro@...tes.org>, "afael@...nel.org"
	<afael@...nel.org>, "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"alex@...zbot.org" <alex@...zbot.org>, "jgg@...dia.com" <jgg@...dia.com>,
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	<robin.murphy@....com>, "lenb@...nel.org" <lenb@...nel.org>,
	"baolu.lu@...ux.intel.com" <baolu.lu@...ux.intel.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-acpi@...r.kernel.org"
	<linux-acpi@...r.kernel.org>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"patches@...ts.linux.dev" <patches@...ts.linux.dev>, "Jaroszynski, Piotr"
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	"helgaas@...nel.org" <helgaas@...nel.org>, "etzhao1900@...il.com"
	<etzhao1900@...il.com>
Subject: RE: [PATCH v5 5/5] pci: Suspend iommu function prior to resetting a
 device

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Tuesday, November 18, 2025 9:42 AM
> 
> On Tue, Nov 18, 2025 at 12:29:43AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Tuesday, November 18, 2025 3:27 AM
> > >
> > > On Mon, Nov 17, 2025 at 04:52:05AM +0000, Tian, Kevin wrote:
> > > > > From: Nicolin Chen <nicolinc@...dia.com>
> > > > > Sent: Saturday, November 15, 2025 2:01 AM
> > > > >
> > > > > On Fri, Nov 14, 2025 at 09:45:31AM +0000, Tian, Kevin wrote:
> > > > > > > From: Nicolin Chen <nicolinc@...dia.com>
> > > > > > > Sent: Tuesday, November 11, 2025 1:13 PM
> > > > > > >
> > > > > > > +/*
> > > > > > > + * Per PCIe r6.3, sec 10.3.1 IMPLEMENTATION NOTE, software
> > > disables
> > > > > ATS
> > > > > > > before
> > > > > > > + * initiating a reset. Notify the iommu driver that enabled ATS.
> > > > > > > + */
> > > > > > > +int pci_reset_iommu_prepare(struct pci_dev *dev)
> > > > > > > +{
> > > > > > > +	if (pci_ats_supported(dev))
> > > > > > > +		return iommu_dev_reset_prepare(&dev->dev);
> > > > > > > +	return 0;
> > > > > > > +}
> > > > > >
> > > > > > the comment says "driver that enabled ATS", but the code checks
> > > > > > whether ATS is supported.
> > > > > >
> > > > > > which one is desired?
> > > > >
> > > > > The comments says "the iommu driver that enabled ATS". It doesn't
> > > > > conflict with what the PCI core checks here?
> > > >
> > > > actually this is sent to all IOMMU drivers. there is no check on whether
> > > > a specific driver has enabled ATS in this path.
> > >
> > > But the comment doesn't say "check"..
> > >
> > > How about "Notify the iommu driver that enables/disables ATS"?
> > >
> > > The point is that pci_enable_ats() is called in iommu drivers.
> > >
> >
> > but in current way even an iommu driver which doesn't call
> > pci_enable_ats() will also be notified then I didn't see the
> > point of adding an attribute to "the iommu driver".
> 
> Hmm, that's a fair point.
> 
> Having looked closely, I see only AMD and ARM call that to enable
> ATs. How others (e.g. Intel) enable it?
> 
> And how do you think of the followings?
> 
> /*
>  * Per PCIe r6.3, sec 10.3.1 IMPLEMENTATION NOTE, software disables ATS
> before
>  * initiating a reset. Though not all IOMMU drivers calls pci_enable_ats(), it
>  * only gets invoked in IOMMU driver. And it is racy to check dev-
> >ats_enabled
>  * here, as a concurrent IOMMU attachment can enable ATS right after this
> line.
>  *
>  * Notify the IOMMU driver to stop IOMMU translations until the reset is
> done,
>  * to ensure that the ATS function and its related invalidations are disabled.
>  */
> 

I'd remove the words between "Though not ..." and "after this line", which
could be explained in iommu side following Bjorn's suggestion to not check
pci_ats_supported() in pci core.