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

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Message-ID: <20240904200325.GA345254@bhelgaas>
Date: Wed, 4 Sep 2024 15:03:25 -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 Wed, Sep 04, 2024 at 02:48:30PM -0500, Wei Huang wrote:
> On 9/4/24 13:49, Bjorn Helgaas wrote:
> > 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.
> 
> I'll incorporate some of these descriptions into the TPH patches where
> relevant. Additionally, I'll enhance the commit log for bnxt.c (patch
> 11) with examples of the benefits.

Sounds good.

Another confusing point that would be helpful to mention is that TPH
includes two pieces: Processing Hints and Steering Tags.

As far as I can see, the only architected control of Processing Hints
(bi-directional, requester, target, target w/ priority) is to
enable/disable TPH or Extended TPH.

But we *do* have significant software control over the Steering Tags.