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Date: Wed, 25 Aug 2010 00:06:34 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Borislav Petkov <borislav.petkov@amd.com>
CC: Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [PATCH] x86: fix CONFIG_VMSPLIT_1G and 2G_OPT trampoline
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On 08/24/2010 11:30 PM, Borislav Petkov wrote:
> From: Hugh Dickins <hughd@google.com>
> Date: Wed, Aug 25, 2010 at 01:44:12AM -0400
> 
>> rc2 kernel crashes when booting second cpu on this CONFIG_VMSPLIT_2G_OPT
>> laptop: whereas cloning from kernel to low mappings pgd range does need
>> to limit by both KERNEL_PGD_PTRS and KERNEL_PGD_BOUNDARY, cloning kernel
>> pgd range itself must not be limited by the smaller KERNEL_PGD_BOUNDARY.
> 
> Yep, makes sense.
> 
> In looking around for other swapper_pg_dir manipulations, there's this
> piece in <arch/x86/kernel/reboot.c:machine_real_restart()>:
> 
>        /* Remap the kernel at virtual address zero, as well as offset zero
>           from the kernel segment.  This assumes the kernel segment starts at
>           virtual address PAGE_OFFSET. */
>        memcpy(swapper_pg_dir, swapper_pg_dir + KERNEL_PGD_BOUNDARY,
>                sizeof(swapper_pg_dir [0]) * KERNEL_PGD_PTRS);
> 
> and we're cloning to low mappings here but still take KERNEL_PGD_PTRS.
> 
> In the CONFIG_VMSPLIT_2G_OPT case, KERNEL_PGD_BOUNDARY and
> KERNEL_PGD_PTRS are 480 and 544, respectively, and IINM, and we end up
> overwriting the virtual mappings beginning at PAGE_OFFSET.
> 
> I guess this doesn't matter on reboot or am I missing something else?
> 

It doesn't matter on reboot, but the right thing would be to switch to
the trampoline page table here.  Same with suspend.

	-hpa

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