linux-kernel - Re: [PATCH] x86/boot: Reorganize and clean up the BIOS area reservation code

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Date:	Thu, 21 Jul 2016 14:28:16 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...nel.org>
CC:	Matt Fleming <mfleming@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mario Limonciello <mario_limonciello@...l.com>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/boot: Reorganize and clean up the BIOS area reservation code

On July 21, 2016 2:08:12 PM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Thu, Jul 21, 2016 at 9:18 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> * Andy Lutomirski <luto@...capital.net> wrote:
>>
>>> It would be very easy to implement this if we could handle
>overlapping memblocks
>>> precisely or set a lower limit on the memblock allocator. Then we
>could block
>>> off everything below 1MB or 2MB very early and then unblock it or
>temporarily
>>> change the lower limit and ask for a single page for the trampoline
>after that.
>>
>> So my suggestion was/is to _permanently_ allocate the SMP trampoline
>page, and
>> leave it also reserved.
>>
>> 'Reserving' a memory area is really just a kernel internal matter. We
>can still
>> use it. No need to unreserve/allocate/re-reserve ... unless I'm
>missing something.
>>
>
>I don't think you're missing anything particularly deep.  I'm just
>talking about an implementation issue.  We need to make sure that the
>page we pick for the trampoline isn't reserved in the memory map or by
>some other quirk (including the EBDA).  The kernel currently uses
>memblock for this, which means that we should probably play nicely
>with the memblock code.
>
>To fix my laptop, though, I think we either need to change the EBDA
>reservation (i.e. be willing to pick a page above the EBDA but below
>the BIOS) or rework the code so that it can use a BOOT_SERVICES_DATA
>page.
>
>--Andy

Oh... this is booting in EFI mode.  Now it makes a bit more sense.  This is a headache because I believe the same company has put out systems which crash if boot services memory is reclaimed before driver initialization, which is the only reason we can't just treat it as plain memory immediately after invoking ExitBootServices.

In theory EBDA in EFI mode is nonsense, too, but not trying to be smart about it might break some systems due to interaction with ACPI.

The real mode trampoline code isn't all that large; it is fundamentally limited to 64K but I think it is about 24K right now.  This is why we have the 128K "insane" threshold: it is still big enough that we can fit.
-- 
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