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

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Message-ID: <66aa72f4-6630-4270-830b-0252f650529e@linux.intel.com>
Date: Thu, 18 Sep 2025 12:35:51 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: 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>, Joel Granados <joel.granados@...nel.org>
Cc: 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 9/15/25 14:29, 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.
> 
> Reported-by: Joel Granados<joel.granados@...nel.org>
> Closes:https://lore.kernel.org/r/20250909-jag-pds-v1-1-ad8cba0e494e@kernel.org
> Fixes: 66ac4db36f4c ("iommu/vt-d: Add page request draining support")
> Cc:stable@...r.kernel.org
> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)

Queued for v6.18.