linux-kernel - Re: [PATCH V4 00/12] PCIe TPH and cache direct injection support

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Message-ID: <20240904184911.GA340610@bhelgaas>
Date: Wed, 4 Sep 2024 13:49:11 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Wei Huang <wei.huang2@....com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, netdev@...r.kernel.org,
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Subject: Re: [PATCH V4 00/12] PCIe TPH and cache direct injection support

On Thu, Aug 22, 2024 at 03:41:08PM -0500, Wei Huang wrote:
> Hi All,
> 
> TPH (TLP Processing Hints) is a PCIe feature that allows endpoint
> devices to provide optimization hints for requests that target memory
> space. These hints, in a format called steering tag (ST), are provided
> in the requester's TLP headers and allow the system hardware, including
> the Root Complex, to optimize the utilization of platform resources
> for the requests.
> 
> Upcoming AMD hardware implement a new Cache Injection feature that
> leverages TPH. Cache Injection allows PCIe endpoints to inject I/O
> Coherent DMA writes directly into an L2 within the CCX (core complex)
> closest to the CPU core that will consume it. This technology is aimed
> at applications requiring high performance and low latency, such as
> networking and storage applications.

Thanks for this example, it's a great intro.  Suggest adding something
similar to a patch commit log, since the cover letter is harder to
find after this appears in git.

> This series introduces generic TPH support in Linux, allowing STs to be
> retrieved and used by PCIe endpoint drivers as needed. As a
> demonstration, it includes an example usage in the Broadcom BNXT driver.
> When running on Broadcom NICs with the appropriate firmware, it shows
> substantial memory bandwidth savings and better network bandwidth using
> real-world benchmarks. This solution is vendor-neutral and implemented
> based on industry standards (PCIe Spec and PCI FW Spec).
> 
> V3->V4:
>  * Rebase on top of the latest pci/next tree (tag: 6.11-rc1)

No need to rebase to pci/next; pci/main is where it will be applied.
But it currently applies cleanly to either, so no problem.

>  * Add new API functioins to query/enable/disable TPH support
>  * Make pcie_tph_set_st() completely independent from pcie_tph_get_cpu_st()
>  * Rewrite bnxt.c based on new APIs
>  * Remove documentation for now due to constantly changing API

I'd like to see this documentation included.  And updated if the API
changes, of course.

>  * Remove pci=notph, but keep pci=nostmode with better flow (Bjorn)

This seems backward to me.  I think "pci=notph" makes sense as a way
to completely disable the TPH feature in case a user trips over a
hardware or driver defect.

But "pci=nostmode" is advertised as a way to quantify the benefit of
Steering Tags, and that seems like it's of interest to developers but
not users.

So my advice would be to keep "pci=notph" and drop "pci=nostmode".

Bjorn