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[173.79.56.208]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7c3e55116a5sm111483085a.106.2025.03.06.09.08.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Mar 2025 09:08:51 -0800 (PST) Date: Thu, 6 Mar 2025 12:08:49 -0500 From: Gregory Price To: Yuquan Wang Cc: lsf-pc@lists.linux-foundation.org, linux-mm@kvack.org, linux-cxl@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [LSF/MM] CXL Boot to Bash - Section 0: ACPI and Linux Resources Message-ID: References: Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Mar 06, 2025 at 09:37:49AM +0800, Yuquan Wang wrote: > On Wed, Mar 05, 2025 at 05:20:52PM -0500, Gregory Price wrote: First, thank you for bringing this up, this is exactly the type of ambiguiuty i was hoping others would contribute. It's difficult to figure out if the ACPI tables are "Correct", if there's unimplemented features, or we're doing something wrong - because some of this is undocumented theory of operation. > > ================== > > NUMA node creation > > =================== > > NUMA nodes are *NOT* hot-pluggable. All *POSSIBLE* NUMA nodes are > > identified at `__init` time, more specifically during `mm_init`. > > > > What this means is that the CEDT and SRAT must contain sufficient > > `proximity domain` information for linux to identify how many NUMA > > nodes are required (and what memory regions to associate with them). > > > Condition: > 1) A UMA/NUMA system that SRAT is absence, but it keeps CEDT.CFMWS > 2）Enable CONFIG_ACPI_NUMA > > Results: > 1) acpi_numa_init: the fake_pxm will be 0 and send to acpi_parse_cfmws() > 2）If dynamically create cxl ram region, the cxl memory would be assigned > to node0 rather than a fake new node. > This is very interesting. Can I ask a few questions: 1) is this real hardware or a VM? 2) By `dynamic creation` you mean leveraging cxl-cli (ndctl)? 2a) Is the BIOS programming decoders, or are you programming the decoder after boot? > Confusions: > 1) Does CXL memory usage require a numa system with SRAT? As you > mentioned in SRAT section: > > "This table is technically optional, but for performance information > to be enumerated by linux it must be present." > > Hence, as I understand it, it seems a bug in kernel. > It's hard to say if this is a bug yet. It's either a bug, or your system should have an SRAT to describe what the BIOS has done. > 2) If it is a bug, could we forbid this situation by adding fake_pxm > check and returning error in acpi_numa_init()? > > 3）If not, maybe we can add some kernel logic to allow create these fake > nodes on a system without SRAT? > I think we should at least provide a warning (if the SRAT is expected but missing) - but lets get some more information first. ~Gregory