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

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Message-ID: <zkgvbw42g25a47nyydehxismaup6eh4kygqbdw7fk54kxze7j3@lrczardwx2ma>
Date: Mon, 15 Sep 2025 13:30:56 +0200
From: Joel Granados <joel.granados@...nel.org>
To: Lu Baolu <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 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?
> 
> 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(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 9c3ab9d9f69a..92759a8f8330 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3812,7 +3812,7 @@ static struct iommu_device *intel_iommu_probe_device(struct device *dev)
>  			}
>  
>  			if (info->ats_supported && ecap_prs(iommu->ecap) &&
> -			    pci_pri_supported(pdev))
> +			    ecap_pds(iommu->ecap) && pci_pri_supported(pdev))
Should this be
 +			    ecap_dev_iotlb_support(iommu->ecap) && ecap_pds(iommu->ecap) && pci_pri_supported(pdev))

???

best
-- 

Joel Granados

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