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: <8ab86203-ce4e-464d-81ea-2425e769d8a1@linux.intel.com>
Date: Wed, 4 Sep 2024 09:37:35 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Joel Granados <j.granados@...sung.com>
Cc: baolu.lu@...ux.intel.com, Jason Gunthorpe <jgg@...pe.ca>,
 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 9/3/24 9:20 PM, Joel Granados wrote:
> On Mon, Sep 02, 2024 at 08:47:21PM +0800, Baolu Lu wrote:
>> 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.
>>
> Thx for getting back to me. This generates another doubt in my head
> regarding the published capabilities from the intel IOMMU Hardware:
> 
> So ecap_pasid [1] does not have to be set in scalable-mode. Right? This
> allows hardware supporting scalable-mode to reject transactions with
> PASID whenever ecap_pasid is*NOT*  set; even though internally things
> are handled with a PASID. This question is directly related to the two
> last patches in the set.5/6 and 6/6.

Yes. And 5/6, 6/6 make sense to me. We should remove the PASID
restriction from the code once PRI is split from SVA.

Thanks,
baolu