Re: [Buildroot] [PATCH] package/qemu: host-qemu does not support AVX512 CPUs

From: Thomas Petazzoni via buildroot <buildroot@buildroot.org>
To: Yegor Yefremov <yegorslists@googlemail.com>
Cc: buildroot@buildroot.org, Romain Naour <romain.naour@gmail.com>
Subject: Re: [Buildroot] [PATCH] package/qemu: host-qemu does not support AVX512 CPUs
Date: Wed, 16 Sep 2026 20:57:11 +0200	[thread overview]
Message-ID: <aqrl-vOwcHy1wpBs@windsurf> (raw)
In-Reply-To: <20260914092612.1283030-1-yegorslists@gmail.com>

Hello,

Thanks for the patch!

On Mon, Sep 14, 2026 at 11:26:12AM +0200, Yegor Yefremov via buildroot wrote:
> From: Yegor Yefremov <yegorslists@googlemail.com>
> 
> Building gobject-introspection for an x86 CPU variant with AVX512 fails
> when g-ir-scanner runs the freshly built target binaries under
> qemu-user:
> 
>   qemu: uncaught target signal 4 (Illegal instruction) - core dumped
> 
> The Qemu TCG engine implements AVX, AVX2, F16C, FMA3 and VAES since Qemu
> 7.2, but it does not implement the AVX512 instruction set at all [0].
> Therefore no -cpu value passed through
> BR2_PACKAGE_HOST_QEMU_USER_MODE_ARGS can make such binaries run, and
> user-mode emulation is simply not possible for these CPU variants.
> 
> Mark those CPUs as unsupported by host-qemu, the same way it is already
> done for x86_steamroller and x86_core_avx2. This relies on the existing

Does our BR2_x86_core_avx2 exclusion still makes sense?

1. You're mentioning that the TCG engine implements AVX2, so why is
supporting x86_core_avx2 a problem?

2. According to arch/Config.in.x86, BR2_x86_core_avx2 is deprecated
and BR2_x86_haswell should be used instead, which I suppose means they
are equivalent. But we do exclude BR2_x86_core_avx2 but not
BR2_x86_haswell in BR2_PACKAGE_HOST_QEMU_ARCH_SUPPORTS, which doesn't
make much sense.

What do you think?

> BR2_X86_CPU_HAS_AVX512 symbol, so all AVX512 capable variants are
> covered, and it disables gobject-introspection and nodejs, which are the
> two packages needing host-qemu in user mode.
> 
> [0] https://gitlab.com/qemu-project/qemu/-/issues/2878
> 
> Fixes:
> 
>   https://autobuild.buildroot.org/results/7ed7f9c36dae1ddb965a0f0021db6cd319927b47/
> 
> Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
> Assisted-by: Claude:claude-opus-5

Nevertheless, your patch is good by itself, so I've applied it to
master. Thanks!

Thomas
-- 
Thomas Petazzoni, co-owner and CEO, Bootlin
Embedded Linux and Kernel engineering and training
https://bootlin.com
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