linux-kernel - Re: [PATCH V6 0/5] PCIe TPH and cache direct injection support

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Message-ID: <20240930165519.GA179473@bhelgaas>
Date: Mon, 30 Sep 2024 11:55:19 -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,
	Jonathan.Cameron@...wei.com, corbet@....net, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	alex.williamson@...hat.com, gospo@...adcom.com,
	michael.chan@...adcom.com, ajit.khaparde@...adcom.com,
	somnath.kotur@...adcom.com, andrew.gospodarek@...adcom.com,
	manoj.panicker2@....com, Eric.VanTassell@....com,
	vadim.fedorenko@...ux.dev, horms@...nel.org, bagasdotme@...il.com,
	bhelgaas@...gle.com, lukas@...ner.de, paul.e.luse@...el.com,
	jing2.liu@...el.com
Subject: Re: [PATCH V6 0/5] PCIe TPH and cache direct injection support

On Fri, Sep 27, 2024 at 04:56:48PM -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.
> 
> 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).
> 
> V5->V6:
>  * Rebase on top of pci/main (tag: pci-v6.12-changes)
>  * Fix spellings and FIELD_PREP/bnxt.c compilation errors (Simon)
>  * Move tph.c to drivers/pci directory (Lukas)
>  * Remove CONFIG_ACPI dependency (Lukas)
>  * Slightly re-arrange save/restore sequence (Lukas)

Thanks, I'll wait for the kernel test robot warnings to be resolved.

In patch 2/5, reword commit logs as imperative mood, e.g.,
s/X() is added/Add X()/, as you've already done for 1/5 and 3/5.

Maybe specify the ACPI _DSM name?  This would help users know whether
their system can use this, or help them request that a vendor
implement the _DSM.

In patch 4/5, s/sustancial/substantial/.  I guess the firmware you
refer to here means the system firmware that would provide the _DSM
required for this to work, i.e., not firmware on the NIC itself?
Would be helpful for users to have a hint as to how to tell whether to
expect a benefit on their system.

The 5/5 commit log could say what the patch *does*, not what *could*
be done (the subject does say what the patch does, but it's nice if
it's in the commit log as well so it's complete by itself).  Also, a
hint that using the steering tag helps direct DMA writes to a cache
close to the CPU expected to consume it might be helpful to motivate
the patch.

Bjorn