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

public inbox for [email protected]  
help / color / mirror / Atom feed

From: Greg Burd <[email protected]>
To: Thomas Munro <[email protected]>
To: Nathan Bossart <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: Dave Cramer <[email protected]>
Subject: Re: [PATCH] Fix ARM64/MSVC atomic memory ordering issues on Win11 by adding explicit DMB ?barriers
Date: Wed, 17 Dec 2025 13:34:13 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <CA+hUKG+-ESmkaDZ4a_S--UoJ2soVRgsa6uem7REM7LuYxk5xFg@mail.gmail.com>
References: <aTw8tb_eIpDBYHfW@nathan>
	<[email protected]>
	<sxhkd5ucchq4vululj6xiwkrd6tsxh53gjiku2knjnqalglumc@hs4vp3b7e7by>
	<[email protected]>
	<aUB_6ZNFRtjM7FVS@nathan>
	<qu3uuja2u3uerwmbhvz6pscdhlhdm3xa3d4nc7lr7vvpowarzl@yn45ne6635ml>
	<aUCTH8oP4Wj4r58P@nathan>
	<[email protected]>
	<[email protected]>
	<aUGf5oyzguybiUBB@nathan>
	<aUHR3eSjkaBjaUAB@nathan>
	<CA+hUKG+-ESmkaDZ4a_S--UoJ2soVRgsa6uem7REM7LuYxk5xFg@mail.gmail.com>


On Tue, Dec 16, 2025, at 8:57 PM, Thomas Munro wrote:
> On Wed, Dec 17, 2025 at 10:40 AM Nathan Bossart
> <[email protected]> wrote:

Hey Nathan, Thomas, thanks for your continued investment of time in this patch.  More thoughts below, but I wonder if the thing to do is to commit this and then for me to follow-on with a few more targeted changes to round out this platform combo?

>> 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.

I think it would be amazing to support this eventually, but as I'm not a MSVC or Win11 guru this feels like a longer term patch.  I'm still struggling with simpler things at the moment.  The platform is not well established so some libraries are missing or out of date.  There's no ARM64-native Perl either.

>> > 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.

When I get a chance I'll post a docs patch for debate/review.

>> 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.

I think there are a few things to offer up in the next few weeks:
* docs
* popcnt
* SIMD
* wiki page with HOWTO info for the next "brave" soul to give this a go
* what else?

What this patch does is get "unicorn" off the dead list, which would be nice.

-greg

view thread (42+ messages)

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]
  Subject: Re: [PATCH] Fix ARM64/MSVC atomic memory ordering issues on Win11 by adding explicit DMB ?barriers
  In-Reply-To: <[email protected]>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox