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Date:   Tue, 8 Jun 2021 22:01:50 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Michael Kelley <mikelley@microsoft.com>,
        James Morris <jmorris@namei.org>,
        Sasha Levin <sashal@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>, x86-ml <x86@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        James Morris <James.Morris@microsoft.com>
Subject: Re: [GIT PULL] x86/urgent for v5.13-rc5
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On Tue, Jun 08, 2021 at 12:22:50PM -0700, Linus Torvalds wrote:
> On Tue, Jun 8, 2021 at 11:33 AM Borislav Petkov <bp@alien8.de> wrote:
> >
> > Linus, maybe we should at least give it a try and see whether someone
> > complains and revert, potentially...?
> 
> I already merged the change that did that.

Oh yeah, I meant: let's leave it in and try it and only revert if there's
trouble.

> It might be worth adding a comment about the verified Windows behavior
> here, but I think otherwise we're all done.

How's that (comment re-flowed):

diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 1e720626069a..b58cdc70f86b 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1084,17 +1084,18 @@ 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();
 

Thx.

-- 
Regards/Gruss,
    Boris.

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