linux-kernel - Re: [PATCH v1 15/29] cxl/region: Use an endpoint's SPA range to find a region

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Message-ID: <Z319Bj1Nk5LAZoL0@gourry-fedora-PF4VCD3F>
Date: Tue, 7 Jan 2025 14:14:14 -0500
From: Gregory Price <gourry@...rry.net>
To: Robert Richter <rrichter@....com>
Cc: Alison Schofield <alison.schofield@...el.com>,
	Vishal Verma <vishal.l.verma@...el.com>,
	Ira Weiny <ira.weiny@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Dave Jiang <dave.jiang@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, linux-cxl@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	"Fabio M. De Francesco" <fabio.m.de.francesco@...ux.intel.com>,
	Terry Bowman <terry.bowman@....com>
Subject: Re: [PATCH v1 15/29] cxl/region: Use an endpoint's SPA range to find
 a region

On Tue, Jan 07, 2025 at 03:10:01PM +0100, Robert Richter wrote:
> To find the correct region and root port of an endpoint of a system
> needing address translation, the endpoint's HPA range must be
> translated to each of the parent port address ranges up to the root
> decoder.
> 
> Calculate the SPA range using the newly introduced callback function
> port->to_hpa() that translates the decoder's HPA range to its parent
> port's HPA range of the next outer memory domain. Introduce the helper
> function cxl_port_calc_hpa() for this to calculate address ranges
> using the low-level port->to_hpa() callbacks. Determine the root port
> SPA range by iterating all the ports up to the root. Store the
> endpoint's SPA range and use it to find the endpoint's region.
>

Some comments/questions inline.

> Signed-off-by: Robert Richter <rrichter@....com>
> ---
>  drivers/cxl/core/region.c | 85 ++++++++++++++++++++++++++++++++-------
>  drivers/cxl/cxl.h         |  1 +
>  2 files changed, 71 insertions(+), 15 deletions(-)
... snip ...
> +	while (1) {
> +		parent = parent_port_of(iter);
> +

I'm always suspicious of unbounded while loops.  Should this simply be

	while(iter) {
		...
	}

Instead?  Given the rest of the function, I don't think this matters,
but it's at least a bit clearer.

> +		if (is_cxl_endpoint(iter))
> +			cxld = &cxled->cxld;
> +		else if (!parent || parent->to_hpa)
> +			cxld = cxl_port_find_switch_decoder(iter, &hpa);
> +
> +		if (!cxld) {
> +			dev_err(cxlmd->dev.parent,
> +				"%s:%s no CXL window for range %#llx:%#llx\n",
> +				dev_name(&cxlmd->dev), dev_name(&cxled->cxld.dev),
> +				hpa.start, hpa.end);
> +			return -ENXIO;
> +		}

I also think we should be able to move this check out of the loop on
various break conditions (iter==NULL, cxld not found, etc).

> +
> +		/* No parent means the root port was found. */
> +		if (!parent)
> +			break;
> +
> +		/* Translate HPA to the next upper memory domain. */
> +		if (cxl_port_calc_hpa(parent, cxld, &hpa))
> +			return -ENXIO;
> +
> +		iter = parent;
>  	}
>
> +	dev_dbg(cxld->dev.parent,
> +		"%s:%s: range:%#llx-%#llx\n",
> +		dev_name(&cxled->cxld.dev), dev_name(&cxld->dev),
> +		hpa.start, hpa.end);
> +
>  	cxled->cxlrd = to_cxl_root_decoder(&cxld->dev);
> +	cxled->spa_range = hpa;
>  
>  	return 0;
>  }

Overall good, just think we might be able to improve the readability /
safety of this loop a bit.

Stupid question: I presume a look in this iteration is not (generally?)
possible, but if it were to happen this while loop as-is would go infinite.
Is that something we should worry about it?

~Gregory