Re: [PATCH] build: prefer sigaltstack over ucontext on Linux

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Apr 24, 2026 at 11:19:39AM +0300, Elisey Konstantinov wrote:
> From 517099e65de1e2ad3d0f33d3c4a7a71f8728279c Mon Sep 17 00:00:00 2001
> From: Elisey <elisey.konstantinov@gmail.com>
> Date: Mon, 23 Mar 2026 16:02:45 +0100
> Subject: [PATCH] build: prefer sigaltstack over ucontext on Linux
> 
> sigaltstack coroutine backend is more stable on some Linux
> configurations, while ucontext causes boot failures for certain
> PPC Mac OS X guests (issue #3276).
> Select sigaltstack first when available, keeping ucontext as fallback
> until the underlying issue is resolved.

IMHO there needs to be a greater investigation in to why ucontext
would be demonstrating the failure. ucontext has been the default
on Linux for QEMU for decades without significant issues, so my
concern is that switching to sigaltstack is not fixing the root
cause of the bug you see, just masking it by luck.

The choice of coroutine backend can also be performance sensitive
and so changing it is something that would need to also bring
along performance benchmark data to illustrate the likely impact
of the change.

> 
> Signed-off-by: Elisey Konstantinov <elisey.konstantinov@gmail.com>
> ---
>  meson.build | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/meson.build b/meson.build
> index ab3e97eb9f..742655e007 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -505,10 +505,12 @@ if host_os == 'windows'
>  elif host_os == 'emscripten'
>    supported_backends += ['wasm']
>  else
> +  # FIXME: This is a temporary workaround!
> +  # SigAltStack is in higher priority to fix #3276
> +  supported_backends += ['sigaltstack']
>    if host_os != 'darwin' and cc.links(ucontext_probe)
>      supported_backends += ['ucontext']
>    endif
> -  supported_backends += ['sigaltstack']
>  endif
>  
>  if coroutine_backend == 'auto'
> -- 
> 2.50.1 (Apple Git-155)
>