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

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Message-ID: <Y3vMpbsHA35VoasD@zn.tnic>
Date:   Mon, 21 Nov 2022 20:08:21 +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>
Subject: Re: [PATCH] x86/boot: skip realmode init code when running as Xen PV
 guest

On Mon, Nov 21, 2022 at 05:24:33PM +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).
> 
> Skip the trampoline setup code for Xen PV guests.
> 
> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/include/asm/realmode.h | 4 ++--
>  arch/x86/realmode/init.c        | 3 +++
>  2 files changed, 5 insertions(+), 2 deletions(-)
 
> diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
> index fd6f6e5b755a..5bfce58f1bab 100644
> --- a/arch/x86/include/asm/realmode.h
> +++ b/arch/x86/include/asm/realmode.h
> @@ -78,8 +78,8 @@ extern unsigned char secondary_startup_64_no_verify[];
>  
>  static inline size_t real_mode_size_needed(void)
>  {
> -	if (real_mode_header)
> -		return 0;	/* already allocated. */
> +	if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
> +		return 0;	/* already allocated or not needed. */
>  
>  	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
>  }
> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> index 41d7669a97ad..1826700b156e 100644
> --- a/arch/x86/realmode/init.c
> +++ b/arch/x86/realmode/init.c
> @@ -202,6 +202,9 @@ static void __init set_real_mode_permissions(void)
>  
>  static int __init init_real_mode(void)
>  {
> +	if (cpu_feature_enabled(X86_FEATURE_XENPV))a

This reminds me of the notorious if (xen) sprinkling from years ago.
Please don't do that.

-- 
Regards/Gruss,
    Boris.

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