Re: [PATCH v2 10/10] PCI/DOE: Relax restrictions on request and response size

From: Lukas Wunner <lukas@wunner.de>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Jonathan Cameron <Jonathan.Cameron@Huawei.com>,
	linux-pci@vger.kernel.org,
	Gregory Price <gregory.price@memverge.com>,
	Ira Weiny <ira.weiny@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Alison Schofield <alison.schofield@intel.com>,
	Vishal Verma <vishal.l.verma@intel.com>,
	Dave Jiang <dave.jiang@intel.com>,
	"Li, Ming" <ming4.li@intel.com>, Hillf Danton <hdanton@sina.com>,
	Ben Widawsky <bwidawsk@kernel.org>,
	linuxarm@huawei.com, linux-cxl@vger.kernel.org
Subject: Re: [PATCH v2 10/10] PCI/DOE: Relax restrictions on request and response size
Date: Fri, 10 Feb 2023 23:10:57 +0100	[thread overview]
Message-ID: <20230210221057.GA9868@wunner.de> (raw)
In-Reply-To: <20230124235155.GA1114594@bhelgaas>

On Tue, Jan 24, 2023 at 05:51:55PM -0600, Bjorn Helgaas wrote:
> On Tue, Jan 24, 2023 at 12:43:15PM +0000, Jonathan Cameron wrote:
> > On Mon, 23 Jan 2023 11:20:00 +0100 Lukas Wunner <lukas@wunner.de> wrote:
> > > An upcoming user of DOE is CMA (Component Measurement and Authentication,
> > > PCIe r6.0 sec 6.31).
> > > 
> > > It builds on SPDM (Security Protocol and Data Model):
> > > https://www.dmtf.org/dsp/DSP0274
> > > 
> > > SPDM message sizes are not always a multiple of dwords.  To transport
> > > them over DOE without using bounce buffers, allow sending requests and
> > > receiving responses whose final dword is only partially populated.
[...]
> > IIRC, at the time feedback was strongly in favour of making
> > the handling of non dword payloads a problem for the caller (e.g. PCI/CMA)
> > not the DOE core, mainly so that we could keep the layering simple.
> > DOE part of PCI spec says DWORD multiples only, CMA has non dword
> > entries.
> 
> I can't remember, but I might have been the voice in favor of making
> it the caller's problem.  Your argument about dealing with it here
> makes a lot of sense, and I'm OK with it, but I *would* like to add
> some text to the commit log and comments in the code about what is
> happening here.  Otherwise there's an unexplained disconnect between
> the DWORD spec language and the byte-oriented code.

In v3 I amended both the commit message and the kerneldoc for pci_doe()
to make it clear that support for arbitrary-sized request and response
buffers is not stipulated by the spec, but merely for the convenience
of the caller.

Thanks,

Lukas