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SubjectRe: [PATCH v11 04/13] PCI: Allow PASID only when ACS enforced on upstreaming path
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On 2022/8/19 07:00, Bjorn Helgaas wrote:
> On Thu, Aug 18, 2022 at 07:53:15PM +0800, Baolu Lu wrote:
>> On 2022/8/18 05:17, Bjorn Helgaas wrote:
>>> On Wed, Aug 17, 2022 at 09:20:15AM +0800, Lu Baolu wrote:
>>>> Some configurations of the PCI fabric will route device originated TLP
>>>> packets based on the memory addresses.
>>> This makes it sound like a few unusual configurations will route TLPs
>>> based on memory addresses, but address routing is the default for all
>>> PCIe Memory Requests, and ACS provides a way to override that default.
>>>
>>>> These configurations are incompatible with PASID as the PASID
>>>> packets form a distinct address space.
>>> I would say "the Requester ID/PASID combination forms a distinct
>>> address space."
>>>
>>>> For instance, any configuration where switches are present
>>>> without ACS enabled is incompatible.
>>>>
>>>> This enhances the pci_enable_pasid() interface by requiring the ACS to
>>>> support Source Validation, Request Redirection, Completer Redirection,
>>>> and Upstream Forwarding. This effectively means that devices cannot
>>>> spoof their requester ID, requests and completions cannot be redirected,
>>>> and all transactions are forwarded upstream, even as it passes through a
>>>> bridge where the target device is downstream.
>>> I think your patch actually requires all those features to be not just
>>> "supported" but actually*enabled* for the entire path leading to the
>>> device.
>>>
>>> To use the terms from the spec:
>>>
>>> "P2P Request Redirect"
>>> "P2P Completion Redirect"
>>> "Requester ID, Requests, and Completions"
>>>
>>> and maybe something like:
>>>
>>> ... even if the TLP looks like a P2P Request because its memory
>>> address (ignoring the PASID) would fall in a bridge window and would
>>> normally be routed downstream.
>> Thank you for the suggestions. I will rephrase the commit message
>> accordingly like this:
>>
>>
>> PCI: Allow PASID only when ACS enforced on upstreaming path
> PCI: Enable PASID only when ACS RR & UF enabled on upstream path
>
> The Requester ID/Process Address Space ID (PASID) combination
> identifies an address space distinct from the PCI bus address space,
> e.g., an address space defined by an IOMMU.
>
> But the PCIe fabric routes Memory Requests based on the TLP address,
> ignoring any PASID (PCIe r6.0, sec 2.2.10.4), so a TLP with PASID that
> *should* go upstream to the IOMMU may instead be routed as a P2P
> Request if its address falls in a bridge window.
>
> To ensure that all Memory Requests with PASID are routed upstream,
> only enable PASID if ACS P2P Request Redirect and Upstream Forwarding
> are enabled for the path leading to the device.

Yours is clear and straight-forward. I will update the patch with above.
Thank you and very appreciated!

Best regards,
baolu

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