linux-kernel - Re: [PATCH 1/1] iommu/vt-d: PRS isn't usable if PDS isn't supported

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Message-ID: <t5ier7jfq7f6sedqq2vholm4x7myolk7tanhi7cjcd6xkrk72d@luay37lq7ipu>
Date: Fri, 19 Sep 2025 09:13:16 +0200
From: Joel Granados <joel.granados@...nel.org>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, 
	Robin Murphy <robin.murphy@....com>, Kevin Tian <kevin.tian@...el.com>, 
	Jason Gunthorpe <jgg@...dia.com>, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH 1/1] iommu/vt-d: PRS isn't usable if PDS isn't supported

On Tue, Sep 16, 2025 at 09:58:14AM +0800, Baolu Lu wrote:
> On 9/15/25 19:30, Joel Granados wrote:
> > On Mon, Sep 15, 2025 at 02:29:46PM +0800, Lu Baolu wrote:
> > > The specification, Section 7.10, "Software Steps to Drain Page Requests &
> > > Responses," requires software to submit an Invalidation Wait Descriptor
> > > (inv_wait_dsc) with the Page-request Drain (PD=1) flag set, along with
> > > the Invalidation Wait Completion Status Write flag (SW=1). It then waits
> > > for the Invalidation Wait Descriptor's completion.
> > > 
> > > However, the PD field in the Invalidation Wait Descriptor is optional, as
> > > stated in Section 6.5.2.9, "Invalidation Wait Descriptor":
> > > 
> > > "Page-request Drain (PD): Remapping hardware implementations reporting
> > >   Page-request draining as not supported (PDS = 0 in ECAP_REG) treat this
> > >   field as reserved."
> > > 
> > > This implies that if the IOMMU doesn't support the PDS capability, software
> > > can't drain page requests and group responses as expected.
> > > 
> > > Do not enable PCI/PRI if the IOMMU doesn't support PDS.
> > 
> > After giving the spec another look, this is probably the way to go.
> > However the PDS also mentions that DT must be set. Should we check
> > ecap_dev_iotlb_support(iommu->ecap)  as well?
> 
> It has already been checked.
Yes. In intel_iommu_probe_device. Thx for the clarification.

Best

-- 

Joel Granados

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