linux-kernel - Re: [PATCH 3/4] x86: Only direct map addresses that are marked as E820

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Date:	Thu, 23 Aug 2012 08:39:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jacob Shin <jacob.shin@....com>
CC:	X86-ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Tejun Heo <tj@...nel.org>,
	Borislav Petkov <borislav.petkov@....com>,
	Dave Young <dyoung@...hat.com>,
	Chao Wang <chaowang@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH 3/4] x86: Only direct map addresses that are marked as
 E820_RAM

On 08/23/2012 07:50 AM, Jacob Shin wrote:
>>
>> I have one concern with this, which is that it leaves in place mapping
>> below the initial max_pfn_mapped.  Although that neatly resolves the
>> legacy area (0-1 MiB) issues, it really isn't right above the 1 MiB
>> point.  Any way I could get you to seek out and unmap any such ranges?
>> We have already seen some Dell machines which put memory holes in low
>> RAM, and perhaps there are still some machines out there with an I/O
>> hole at 15 MiB.
>
> So I believe in V2 of the patchset this was done, however, Dave Young
> from redhat reported that it broke their KVM guest with a user supplied
> memory map that looked like this:
>
>>> [    0.000000] e820: user-defined physical RAM map:
>>> [    0.000000] user: [mem 0x0000000000010000-0x000000000009dbff] usable
>>> [    0.000000] user: [mem 0x0000000024000000-0x0000000033f6bfff] usable
>
> And looking into that scenario, the early boot code seems to allocates
> space for fixmap right under initial max_pfn_mapped, which is no longer
> direct mapped with my patch, and that seems to cause problems for later
> APIC code that initializes APIC base address into the fixmap area.
>
> So I guess to address your concern, we need to go back to V2 and try to
> resolve the fixmap problem with user supplied memory map that reserves
> memory below initial max_pfn_mapped ?
>

Okay... I think I need to grok that a bit better.  For memory 
allocations, we probably should just use brk allocations, for virtual 
space allocations it is called the fixmap for a reason (even though the 
Xen people managed to break that on 32 bits, sigh!)

I guess I need to go back and look at David's bug report...

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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