linux-kernel - Re: [PATCH RFC PREVIEW 0/6] iommu: enable user space iopfs in non-nested and non-svm cases

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Message-ID: <d1e1370a-0714-4da8-b645-f56d83ab0159@linux.intel.com>
Date: Mon, 2 Sep 2024 20:47:21 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Joel Granados <j.granados@...sung.com>, Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, Klaus Jensen <its@...elevant.dk>,
 David Woodhouse <dwmw2@...radead.org>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>, Minwoo Im <minwoo.im@...sung.com>,
 linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
 Klaus Jensen <k.jensen@...sung.com>
Subject: Re: [PATCH RFC PREVIEW 0/6] iommu: enable user space iopfs in
 non-nested and non-svm cases

On 2024/9/2 18:48, Joel Granados wrote:
>> I definitely expect PRI to work outside PASID and SVA cases, so this
>> is going in a good direction
> This touches on a detail (at least in Intel's vtd-io spec) that is not
> 100% clear to me. Second paragraph of section "3.4.3 Scalable Mode
> Address Translation" reads:
> "
>    ... Scalable-mode context-entries support both requests-without-PASID
>    and requests-with-PASID. However unlike legacy mode, in scalable-mode,
>    requests-without-PASID obtain a PASID value from the RID_PASID field of
>    the scalable-mode context- entry and are processed similarly to
>    requests-with-PASID.Implementations not supporting RID_PASID capability
>    (ECAP_REG.RPS is 0b), use a PASID value of 0 to perform address
>    translation for requests without PASID.
> "
> This basically means that a default PASID is used even though the
> request is without PASID. Right? Therefore "outside PASID" means with
> the default PASID (at least in Intels case). Right?

Kind of yes.

The PCI specification defines the concept of PASID and its role in
transaction routing. We refer to PCI transactions with a PASID prefix as
"request-with-PASID" and those without a PASID prefix as "request-
without-PASID." Consequently, I understand 'outside PASID' to mean
transactions that do not have a PASID prefix.

The VT-d specification describes how the IOMMU hardware handles request-
without-PASID. It uses a reserved PASID for its internal routing and
handling purposes. If RID_PASID is supported (ECAP_REG.RPS=1), software
can select its own reserved PASID. Otherwise, the IOMMU hardware will
use a default value of 0.

Thanks,
baolu