linux-kernel - Re: [PATCH] x86/setup: Document that Windows reserves the first MiB

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Date:   Wed, 9 Jun 2021 11:54:39 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        James Morris <jmorris@...ei.org>,
        Sasha Levin <sashal@...nel.org>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        James Morris <James.Morris@...rosoft.com>
Subject: Re: [PATCH] x86/setup: Document that Windows reserves the first MiB

On Tue, Jun 08, 2021 at 10:30:46PM +0200, Borislav Petkov wrote:
> It does so unconditionally too, on Intel and AMD machines, to work
> around BIOS bugs, as confirmed by Microsoft folks (see Link for full
> details).
> 
> Reflow the paragraph, while at it.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>

Acked-by: Mike Rapoport <rppt@...ux.ibm.com>

Thanks for taking care of this!

> Link: https://lkml.kernel.org/r/MWHPR21MB159330952629D36EEDE706B3D7379@MWHPR21MB1593.namprd21.prod.outlook.com
> ---
>  arch/x86/kernel/setup.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
> index 7638ac6c3d80..85acd22f8022 100644
> --- a/arch/x86/kernel/setup.c
> +++ b/arch/x86/kernel/setup.c
> @@ -1060,17 +1060,18 @@ void __init setup_arch(char **cmdline_p)
>  #endif
>  
>  	/*
> -	 * Find free memory for the real mode trampoline and place it
> -	 * there.
> -	 * If there is not enough free memory under 1M, on EFI-enabled
> -	 * systems there will be additional attempt to reclaim the memory
> -	 * for the real mode trampoline at efi_free_boot_services().
> +	 * Find free memory for the real mode trampoline and place it there. If
> +	 * there is not enough free memory under 1M, on EFI-enabled systems
> +	 * there will be additional attempt to reclaim the memory for the real
> +	 * mode trampoline at efi_free_boot_services().
>  	 *
> -	 * Unconditionally reserve the entire first 1M of RAM because
> -	 * BIOSes are know to corrupt low memory and several
> -	 * hundred kilobytes are not worth complex detection what memory gets
> -	 * clobbered. Moreover, on machines with SandyBridge graphics or in
> -	 * setups that use crashkernel the entire 1M is reserved anyway.
> +	 * Unconditionally reserve the entire first 1M of RAM because BIOSes
> +	 * are known to corrupt low memory and several hundred kilobytes are not
> +	 * worth complex detection what memory gets clobbered. Windows does the
> +	 * same thing for very similar reasons.
> +	 *
> +	 * Moreover, on machines with SandyBridge graphics or in setups that use
> +	 * crashkernel the entire 1M is reserved anyway.
>  	 */
>  	reserve_real_mode();
>  
> -- 
> 2.29.2
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

-- 
Sincerely yours,
Mike.