netdev - Re: [PATCH V5 4/6] PCI: Enable 10-Bit tag support for PCIe Endpoint devices

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Date:   Fri, 16 Jul 2021 09:51:28 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dongdong Liu <liudongdong3@...wei.com>,
        Bjorn Helgaas <helgaas@...nel.org>
Cc:     hch@...radead.org, kw@...ux.com, linux-pci@...r.kernel.org,
        rajur@...lsio.com, hverkuil-cisco@...all.nl,
        linux-media@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH V5 4/6] PCI: Enable 10-Bit tag support for PCIe Endpoint
 devices



On 2021-07-16 5:12 a.m., Dongdong Liu wrote:
> Hi Bjorn
> 
> Many thanks for your review.
> 
> On 2021/7/16 1:23, Bjorn Helgaas wrote:
>> [+cc Logan]
>>
>> On Mon, Jun 21, 2021 at 06:27:20PM +0800, Dongdong Liu wrote:
>>> 10-Bit Tag capability, introduced in PCIe-4.0 increases the total Tag
>>> field size from 8 bits to 10 bits.
>>>
>>> For platforms where the RC supports 10-Bit Tag Completer capability,
>>> it is highly recommended for platform firmware or operating software
>>
>> Recommended by whom?  If the spec recommends it, we should provide the
>> citation.
> PCIe spec 5.0 r1.0 section 2.2.6.2 IMPLEMENTATION NOTE says that.
> Will fix.
>>
>>> that configures PCIe hierarchies to Set the 10-Bit Tag Requester Enable
>>> bit automatically in Endpoints with 10-Bit Tag Requester capability. This
>>> enables the important class of 10-Bit Tag capable adapters that send
>>> Memory Read Requests only to host memory.
>>
>> What is the implication for P2PDMA?  What happens if we enable 10-bit
>> tags for device A, and A generates Mem Read Requests to device B,
>> which does not support 10-bit tags?
> PCIe spec 5.0 r1.0 section 2.2.6.2 says
> If an Endpoint supports sending Requests to other Endpoints (as opposed 
> to host memory), the Endpoint must not send 10-Bit Tag Requests to 
> another given Endpoint unless an implementation-specific mechanism 
> determines that the Endpoint supports 10-Bit Tag Completer capability. 
> Not sending 10-Bit Tag Requests to other Endpoints at all
> may be acceptable for some implementations. More sophisticated 
> mechanisms are outside the scope of this specification.
> 
> Not sending 10-Bit Tag Requests to other Endpoints at all seems simple.
> Add kernel parameter pci=pcie_bus_peer2peer when boot kernel with 
> P2PDMA, then do not config 10-BIT Tag.
> 
> if (pcie_bus_config != PCIE_BUS_PEER2PEER)
> 	pci_configure_10bit_tags(dev);
> 
> Bjorn and Logan, any suggestion?

I think we need a check in the P2PDMA code to ensure that a device with
10bit tags doesn't interact with a device that has no 10bit tags. Before
that happens, the kernel should emit a warning saying to enable a
specific kernel parameter.

Though a parameter with a bit more granularity might be appropriate. See
what was done for disable_acs_redir where it affects only the devices
specified in the list.

Thanks,

Logan