Re: [PATCH v6 0/3] memory,x86,acpi: hotplug memory alignment advisement
From: Fan Ni
Date: Wed Nov 20 2024 - 14:20:42 EST
On Wed, Nov 06, 2024 at 10:58:44AM -0500, Gregory Price wrote:
> When physical address regions are not aligned to memory block size,
> the misaligned portion is lost (stranded capacity).
>
> Block size (min/max/selected) is architecture defined. Most architectures
> tend to use the minimum block size or some simplistic heurist. On x86,
> memory block size increases up to 2GB, and is otherwise fitted to the
> alignment of non-hotplug (i.e. not special purpose memory).
>
> CXL exposes its memory for management through the ACPI CEDT (CXL Early
> Detection Table) in a field called the CXL Fixed Memory Window. Per
> the CXL specification, this memory must be aligned to at least 256MB.
>
> When a CFMW aligns on a size less than the block size, this causes a
> loss of up to 2GB per CFMW on x86. It is not uncommon for CFMW to be
> allocated per-device - though this behavior is BIOS defined.
>
> This patch set provides 3 things:
> 1) implement advise/query functions in driverse/base/memory.c to
> report/query architecture agnostic hotplug block alignment advice.
> 2) update x86 memblock size logic to consider the hotplug advice
> 3) add code in acpi/numa/srat.c to report CFMW alignment advice
>
> The advisement interfaces are design to be called during arch_init
> code prior to allocator and smp_init. start_kernel will call these
> through setup_arch() (via acpi and mm/init_64.c on x86), which occurs
> prior to mm_core_init and smp_init - so no need for atomics.
>
> There's an attempt to signal callers to advise() that query has already
> occurred, but this is predicated on the notion that query actually
> occurs (which presently only happens on the x86 arch). This is to
> assist debugging future users. Otherwise, the advise() call has
> been marked __init to help static discovery of bad call times.
>
> Once query is called the first time, it will always return the same value.
>
> Interfaces return -EBUSY and 0 respectively on systems without hotplug.
>
> v6:
> - boot_cpu_has -> cpu_feature_enabled() in x86 code
> - ack tags
>
> Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> Signed-off-by: Gregory Price <gourry@xxxxxxxxxx>
>
Tested on a CXL server with a directly attached cxl device, works as
expected.
Tested-by: Fan Ni <fan.ni@xxxxxxxxxxx>
Fan
> Gregory Price (3):
> memory: implement memory_block_advise/probe_max_size
> x86: probe memory block size advisement value during mm init
> acpi,srat: give memory block size advice based on CFMWS alignment
>
> arch/x86/mm/init_64.c | 15 ++++++++----
> drivers/acpi/numa/srat.c | 12 ++++++++-
> drivers/base/memory.c | 53 ++++++++++++++++++++++++++++++++++++++++
> include/linux/memory.h | 10 ++++++++
> 4 files changed, 84 insertions(+), 6 deletions(-)
>
> --
> 2.43.0
>
--
Fan Ni (From gmail)