linux-kernel - Re: [PATCH v3] x86/boot: skip realmode init code when running as Xen PV guest

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Date:   Thu, 24 Nov 2022 14:28:18 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v3] x86/boot: skip realmode init code when running as Xen
 PV guest

On Wed, Nov 23, 2022 at 12:45:23PM +0100, Juergen Gross wrote:
> When running as a Xen PV guest there is no need for setting up the
> realmode trampoline, as realmode isn't supported in this environment.
> 
> Trying to setup the trampoline has been proven to be problematic in
> some cases, especially when trying to debug early boot problems with
> Xen requiring to keep the EFI boot-services memory mapped (some
> firmware variants seem to claim basically all memory below 1M for boot
> services).
> 
> Introduce new x86_platform_ops operations for that purpose, which can
> be set to a nop by the Xen PV specific kernel boot code.
> 
> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")

This text and Fixes: tag sounds like this needs to go to Linus and
stable now?

> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> index 41d7669a97ad..247aca9f8ed1 100644
> --- a/arch/x86/realmode/init.c
> +++ b/arch/x86/realmode/init.c
> @@ -200,14 +200,18 @@ static void __init set_real_mode_permissions(void)
>  	set_memory_x((unsigned long) text_start, text_size >> PAGE_SHIFT);
>  }
>  
> -static int __init init_real_mode(void)
> +void __init init_real_mode(void)
>  {
>  	if (!real_mode_header)
>  		panic("Real mode trampoline was not allocated");
>  
>  	setup_real_mode();
>  	set_real_mode_permissions();
> +}
>  
> +static int __init call_init_real_mode(void)
> +{
> +	x86_platform.realmode_init();
>  	return 0;
>  }
> -early_initcall(init_real_mode);
> +early_initcall(call_init_real_mode);

I'll name that one "do_init_real_mode" as "call init" sounds weird.

Otherwise, it is as straightforward as it gets.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette