linux-kernel - Re: [PATCH v2] cxl/pci: Fix disabling memory if DVSEC CXL Range does not match a CFMWS window

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Date: Fri, 16 Feb 2024 20:22:58 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Robert Richter <rrichter@....com>, 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>,
	"Dan Williams" <dan.j.williams@...el.com>
CC: Robert Richter <rrichter@....com>, <linux-cxl@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] cxl/pci: Fix disabling memory if DVSEC CXL Range does
 not match a CFMWS window

Robert Richter wrote:
> The Linux CXL subsystem is built on the assumption that HPA == SPA.
> That is, the host physical address (HPA) the HDM decoder registers are
> programmed with are system physical addresses (SPA).
> 
> During HDM decoder setup, the DVSEC CXL range registers (cxl-3.1,
> 8.1.3.8) are checked if the memory is enabled and the CXL range is in
> a HPA window that is described in a CFMWS structure of the CXL host
> bridge (cxl-3.1, 9.18.1.3).
> 
> Now, if the HPA is not an SPA, the CXL range does not match a CFMWS
> window and the CXL memory range will be disabled then. The HDM decoder
> stops working which causes system memory being disabled and further a
> system hang during HDM decoder initialization, typically when a CXL
> enabled kernel boots.
> 
> Prevent a system hang and do not disable the HDM decoder if the
> decoder's CXL range is not found in a CFMWS window.
> 
> Note the change only fixes a hardware hang, but does not implement
> HPA/SPA translation. Support for this can be added in a follow on
> patch series.
> 
> Signed-off-by: Robert Richter <rrichter@....com>
> ---
>  drivers/cxl/core/pci.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
> index a0e7ed5ae25f..18616ca873e5 100644
> --- a/drivers/cxl/core/pci.c
> +++ b/drivers/cxl/core/pci.c
> @@ -478,8 +478,8 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
>  	}
>  
>  	if (!allowed) {
> -		cxl_set_mem_enable(cxlds, 0);
> -		info->mem_enabled = 0;
> +		dev_err(dev, "Range register decodes outside platform defined CXL ranges.\n");
> +		return -ENXIO;

While testing I found this needs the following fixup:

diff --git a/drivers/cxl/core/pci.c b/drivers/cxl/core/pci.c
index e24ffae8135f..e9e6c81ce034 100644
--- a/drivers/cxl/core/pci.c
+++ b/drivers/cxl/core/pci.c
@@ -477,7 +477,7 @@ int cxl_hdm_decode_init(struct cxl_dev_state *cxlds, struct cxl_hdm *cxlhdm,
                allowed++;
        }
 
-       if (!allowed) {
+       if (!allowed && info->mem_enabled) {
                dev_err(dev, "Range register decodes outside platform defined CXL ranges.\n");
                return -ENXIO;
        }


..i.e. Linux should only give up if it does not understand an active
decode region.

Now this SPA/HPA mismatch will still cause problems later in region
creation flow, but that's a separate issue.