linux-kernel - Re: [PATCH 1/2] x86/xen: add CPU dependencies for 32-bit build

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Date:   Thu, 14 Dec 2023 13:01:09 +0100
From:   Alyssa Ross <hi@...ssa.is>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/xen: add CPU dependencies for 32-bit build

On Mon, Dec 04, 2023 at 09:47:01AM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
>
> Xen only supports modern CPUs even when running a 32-bit kernel, and it now
> requires a kernel built for a 64 byte (or larger) cache line:
>
> In file included from <command-line>:
> In function 'xen_vcpu_setup',
>     inlined from 'xen_vcpu_setup_restore' at arch/x86/xen/enlighten.c:111:3,
>     inlined from 'xen_vcpu_restore' at arch/x86/xen/enlighten.c:141:3:
> include/linux/compiler_types.h:435:45: error: call to '__compiletime_assert_287' declared with attribute error: BUILD_BUG_ON failed: sizeof(*vcpup) > SMP_CACHE_BYTES
> arch/x86/xen/enlighten.c:166:9: note: in expansion of macro 'BUILD_BUG_ON'
>   166 |         BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
>       |         ^~~~~~~~~~~~
>
> Enforce the dependency with a whitelist of CPU configurations. In normal
> distro kernels, CONFIG_X86_GENERIC is enabled, and this works fine. When this
> is not set, still allow Xen to be built on kernels that target a 64-bit
> capable CPU.
>
> Fixes: db2832309a82 ("x86/xen: fix percpu vcpu_info allocation")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---

This fixes 32-bit distro kernel builds in Nixpkgs, which regressed
with a recent stable kernel update.  Thanks!

Tested-by: Alyssa Ross <hi@...ssa.is>

>  arch/x86/xen/Kconfig | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
> index 9b1ec5d8c99c..a65fc2ae15b4 100644
> --- a/arch/x86/xen/Kconfig
> +++ b/arch/x86/xen/Kconfig
> @@ -9,6 +9,7 @@ config XEN
>  	select PARAVIRT_CLOCK
>  	select X86_HV_CALLBACK_VECTOR
>  	depends on X86_64 || (X86_32 && X86_PAE)
> +	depends on X86_64 || (X86_GENERIC || MPENTIUM4 || MCORE2 || MATOM || MK8)
>  	depends on X86_LOCAL_APIC && X86_TSC
>  	help
>  	  This is the Linux Xen port.  Enabling this will allow the
> --
> 2.39.2
>

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