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Message-ID: <652a4391-4825-46bf-9f64-52e0ef751dff@amd.com> Date: Mon, 29 Sep 2025 21:06:47 -0700 From: "Koralahalli Channabasappa, Smita" <skoralah@....com> To: "Zhijian Li (Fujitsu)" <lizhijian@...itsu.com>, Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>, "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "nvdimm@...ts.linux.dev" <nvdimm@...ts.linux.dev>, "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>, "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org> 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>, Dan Williams <dan.j.williams@...el.com>, Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown <len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming <ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, Ying Huang <huang.ying.caritas@...il.com>, "Xingtao Yao (Fujitsu)" <yaoxt.fnst@...itsu.com>, Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org>, Nathan Fontenot <nathan.fontenot@....com>, Terry Bowman <terry.bowman@....com>, Robert Richter <rrichter@....com>, Benjamin Cheatham <benjamin.cheatham@....com>, PradeepVineshReddy Kodamati <PradeepVineshReddy.Kodamati@....com> Subject: Re: [RFC PATCH 6/6] cxl/region, dax/hmem: Guard CXL DAX region creation and tighten HMEM deps On 8/31/2025 11:21 PM, Zhijian Li (Fujitsu) wrote: > > > On 22/08/2025 11:42, Smita Koralahalli wrote: >> Prevent cxl_region_probe() from unconditionally calling into >> devm_cxl_add_dax_region() when the DEV_DAX_CXL driver is not enabled. >> Wrap the call with IS_ENABLED(CONFIG_DEV_DAX_CXL) so region probe skips >> DAX setup cleanly if no consumer is present. > > A question came to mind: > > Why is the case of `CXL_REGION && !DEV_DAX_CXL` necessary? It appears to fall back to the hmem driver in that scenario. > If so, could we instead simplify it as follows? > > --- a/drivers/cxl/Kconfig > +++ b/drivers/cxl/Kconfig > @@ -200,6 +200,7 @@ config CXL_REGION > depends on SPARSEMEM > select MEMREGION > select GET_FREE_REGION > + select DEV_DAX_CXL > I’m not entirely sure about the full implications of disabling CXL_REGION when DEV_DAX_CXL is disabled. The primary intent of this patch was to address the scenario where DEV_DAX_HMEM=y and CXL=m, which results in DEV_DAX_CXL being disabled. In that configuration, ownership of the soft-reserved ranges incorrectly falls back to hmem instead of being managed by CXL. This leads to misleading output in /proc/iomem, as I illustrated earlier. That said, as you pointed out, dax_hmem is not exclusive to CXL, so I will drop this patch in v2. The next revision will therefore not cover the case of DEV_DAX_HMEM=y and CXL=m. I would appreciate input on how best to handle this scenario efficiently. Thanks Smita >> >> In parallel, update DEV_DAX_HMEM’s Kconfig to depend on >> !CXL_BUS || (CXL_ACPI && CXL_PCI) || m. This ensures: >> >> Built-in (y) HMEM is allowed when CXL is disabled, or when the full >> CXL discovery stack is built-in. Module (m) HMEM remains always possible. > > Hmm,IIUC, `dax_hmem` isn't exclusively designed for CXL. It could support other special memory types (e.g., HBM). > > Thanks > Zhijian > > > >> >> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com> >> --- >> I did not want to override Dan’s original approach, so I am posting this >> as an RFC. >> >> This patch addresses a corner case when applied on top of Patches 1–5. >> >> When DEV_DAX_HMEM=y and CXL=m, the DEV_DAX_CXL option ends up disabled. >> In that configuration, with Patches 1–5 applied, ownership of the Soft >> Reserved ranges falls back to dax_hmem. As a result, /proc/iomem looks >> like this: >> >> 850000000-284fffffff : CXL Window 0 >> 850000000-284fffffff : region3 >> 850000000-284fffffff : Soft Reserved >> 850000000-284fffffff : dax0.0 >> 850000000-284fffffff : System RAM (kmem) >> 2850000000-484fffffff : CXL Window 1 >> 2850000000-484fffffff : region4 >> 2850000000-484fffffff : Soft Reserved >> 2850000000-484fffffff : dax1.0 >> 2850000000-484fffffff : System RAM (kmem) >> 4850000000-684fffffff : CXL Window 2 >> 4850000000-684fffffff : region5 >> 4850000000-684fffffff : Soft Reserved >> 4850000000-684fffffff : dax2.0 >> 4850000000-684fffffff : System RAM (kmem) >> >> In this case the dax devices are created by dax_hmem, not by dax_cxl. >> Consequently, a "cxl disable-region <regionx>" operation does not >> unregister these devices. In addition, the dmesg output can be misleading >> to users, since it looks like the CXL region driver created the devdax >> devices: >> >> devm_cxl_add_region: cxl_acpi ACPI0017:00: decoder0.2: created region5 >> .. >> .. >> >> This patch addresses those situations. I am not entirely sure how clean >> the approach of using “|| m” is, so I am sending it as RFC for feedback. >> --- >> drivers/cxl/core/region.c | 4 +++- >> drivers/dax/Kconfig | 1 + >> 2 files changed, 4 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c >> index 71cc42d05248..6a2c21e55dbc 100644 >> --- a/drivers/cxl/core/region.c >> +++ b/drivers/cxl/core/region.c >> @@ -3617,7 +3617,9 @@ static int cxl_region_probe(struct device *dev) >> p->res->start, p->res->end, cxlr, >> is_system_ram) > 0) >> return 0; >> - return devm_cxl_add_dax_region(cxlr); >> + if (IS_ENABLED(CONFIG_DEV_DAX_CXL)) >> + return devm_cxl_add_dax_region(cxlr); >> + return 0; >> default: >> dev_dbg(&cxlr->dev, "unsupported region mode: %d\n", >> cxlr->mode); >> diff --git a/drivers/dax/Kconfig b/drivers/dax/Kconfig >> index 3683bb3f2311..fd12cca91c78 100644 >> --- a/drivers/dax/Kconfig >> +++ b/drivers/dax/Kconfig >> @@ -30,6 +30,7 @@ config DEV_DAX_PMEM >> config DEV_DAX_HMEM >> tristate "HMEM DAX: direct access to 'specific purpose' memory" >> depends on EFI_SOFT_RESERVE >> + depends on !CXL_BUS || (CXL_ACPI && CXL_PCI) || m >> select NUMA_KEEP_MEMINFO if NUMA_MEMBLKS >> default DEV_DAX >> help
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