linux-kernel - Re: [PATCH] cxl/pci: Fix disabling CXL memory for zero-based addressing

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Date: Tue, 13 Feb 2024 11:45:48 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Robert Richter <rrichter@....com>, Dan Williams <dan.j.williams@...el.com>
CC: Davidlohr Bueso <dave@...olabs.net>, Jonathan Cameron
	<jonathan.cameron@...wei.com>, Dave Jiang <dave.jiang@...el.com>, "Alison
 Schofield" <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cxl/pci: Fix disabling CXL memory for zero-based
 addressing

Robert Richter wrote:
> On 13.02.24 10:40:07, Dan Williams wrote:
> > Robert Richter wrote:
> > > Dan,
> > > 
> > > On 09.02.24 12:22:01, Dan Williams wrote:
> > > > Robert Richter wrote:
> > > 
> > > > > diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> > > > > index 569354a5536f..3a36a2f0c94f 100644
> > > > > --- a/drivers/cxl/core/pci.c
> > > > > +++ b/drivers/cxl/core/pci.c
> > > > > @@ -466,6 +466,18 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
> > > > >  	for (i = 0, allowed = 0; info->mem_enabled && i < info->ranges; i++) {
> > > > >  		struct device *cxld_dev;
> > > > >  
> > > > > +		/*
> > > > > +		 * Handle zero-based hardware addresses
> > > > > +		 */
> > > > > +		if (!info->dvsec_range[i].start &&
> > > > > +		    info->dvsec_range[i].end != CXL_RESOURCE_NONE &&
> > > > > +		    info->dvsec_range[i].end) {
> > > > > +			dev_dbg(dev, "Zero-based hardware range found [%#llx - %#llx]\n",
> > > > > +				info->dvsec_range[i].start, info->dvsec_range[i].end);
> > > > > +			allowed++;
> > > > > +			continue;
> > > > > +		}
> > > > > +
> > > > 
> > > > I am not comfortable with this. It should be checking a platform
> > > > specific quirk, or similar for the possibility of HPA != SPA. The
> > > > entirety of the Linux CXL subsystem is built on the assumption that HPA
> > > > == SPA, and if a platform wants to inject an offset between those Linux
> > > > needs some way to enumerate that it is running in that new world. Yes,
> > > > nothing in the CXL specification precludes HPA != SPA, but Linux has
> > > > long since shipped the opposite assumption.
> > > 
> > > this check prevents the memory from disabling an enabled decoder. So it
> > > just keeps everything as it comes out of firmware.
> > > 
> > > Can you explain the motivation why active memory is disabled?
> > 
> > It is a sanity check that Linux is operating in a CXL world that it
> > understands. The fundamental assumption is that the CFMWS correctly
> > conveys the CXL space, and that the HW decoder resources match platform
> > expectations match Linux resource management.
> 
> It would be sane to just not use CXL if assumptions on it are not
> valid and not to break system to boot.

I can get on board with that.

> 
> > 
> > > This may take system memory offline and could lead to a kernel hang.
> > 
> > Yes, that is not an unreasonable result when Linux fundamental
> > assumptions are violated.
> 
> BUG_ON(fw_table_broken)? If at all, it is not mandatory to have a
> CFMWS. Btw, the check is more strict and also checks memory
> attributes. It is very likely something can break.

Sure, I'll take a patch like this:

diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
index 6c9c8d92f8f7..e4e5a917f1f4 100644
--- a/drivers/cxl/core/pci.c
+++ b/drivers/cxl/core/pci.c
@@ -477,10 +477,11 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
                allowed++;
        }
 
-       if (!allowed) {
-               cxl_set_mem_enable(cxlds, 0);
-               info->mem_enabled = 0;
-       }
+       WARN_TAINT(!allowed, TAINT_FIRMWARE_WORKAROUND,
+                  FW_BUG "%s: Range register decodes outside platform defined CXL ranges.",
+                  dev_name(dev));
+       if (!allowed)
+               return -ENXIO;
 
        /*
         * Per CXL 2.0 Section 8.1.3.8.3 and 8.1.3.8.4 DVSEC CXL Range 1 Base