Thread

Re: [PATCH] Fix ARM64/MSVC atomic memory ordering issues on Win11 by adding explicit DMB ?barriers

Thomas Munro <thomas.munro@gmail.com> — 2025-12-17T01:57:14Z

On Wed, Dec 17, 2025 at 10:40 AM Nathan Bossart
<nathandbossart@gmail.com> wrote:
> Hm.  I think the USE_LLVM_BACKPORT_SECTION_MEMORY_MANAGER thing might need
> work, too.  We don't have any Windows buildfarm machines with LLVM enabled,
> but IIUC it should be possible.  Perhaps we can add that to unicorn.

The LLVM code has never run on Windows and will likely need
patching... I know of at least one change required and will write
about that.

> > Also, while the patch
> > is targeting Windows 11 (IIUC), there are some notes in the docs that give
> > the impression Windows 10 is supported, too [0].  I could easily change it
> > to say that AArch64 requires Windows 11, but I don't know what to do with
> > the references to specific versions of Visual Studio and the Windows SDK.
>
> Actually, I'm not sure there's anything specific to Windows 11 in this
> patch, besides perhaps the choice to set USE_ARMV8_CRC32C unconditionally.
> I don't know how likely it is that someone will try to run Postgres on
> Windows on an AArch64 machine without CRC extension support, though.

Assuming you haven't blocked OS updates, Windows stopped booting on
pre-ARMv8.1 hardware a while back.  RPi4's Broadcom chip and the
Snapdragon 835 (found in the oldest Windows laptops, according to a
quick Google search) are ARMv8-A only, but did actually have the CRC32
instructions, so it would actually work anyway.  They also have the
optional NEON SIMD stuff, which we use unconditionally:

/*
 * We use the Neon instructions if the compiler provides access to them (as
 * indicated by __ARM_NEON) and we are on aarch64.  While Neon support is
 * technically optional for aarch64, it appears that all available 64-bit
 * hardware does have it.  Neon exists in some 32-bit hardware too, but we
 * could not realistically use it there without a run-time check, which seems
 * not worth the trouble for now.
 */

A single chip in this report lacked FEAT_CRC32, the X-Gene 1 from 2012:

https://gpages.juszkiewicz.com.pl/arm-socs-table/arm-socs.html

So I don't think it's worth worrying about, I was just mentioning the
"Windows 11 requires CRC32, Windows 10 is dead" thing to avoid Greg
being forced to waste time researching the missing feature test code
:-)  The reason Windows can't boot on old ARM chips probably has more
to do with the modern atomics needed for decent lock performance,
which every kernel wants.