Re: [PATCH v11 04/13] PCI: Allow PASID only when ACS enforced on upstreaming path

[Baolu Lu]

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