linux-kernel - Re: [PATCh] x86: overmapped fix when 4K pages on tail

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Date:	Thu, 10 Jul 2008 00:32:12 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Suresh Siddha" <suresh.b.siddha@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Arjan van de Ven" <arjan@...radead.org>
Subject: Re: [PATCh] x86: overmapped fix when 4K pages on tail - 64bit

On Thu, Jul 10, 2008 at 12:20 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Yinghai Lu <yhlu.kernel@...il.com> wrote:
>
>> > that the number of mapping ranges depends on our programming, not on
>> > any external factor. I.e. if anyone adds a new mapping range to the
>> > kernel for any purpose, it must be extended - but otherwise it
>> > cannot run out due to new hardware.
>>
>> 4k, 2M, 1G, 2M, 4k
>>
>> some day will get 512g page?
>
> i'd not be surprised to see that in ~10 years. Then we'll have to extend
> the array to 7 entries ;-)
>
> btw., i have a weird system:
>
> [    0.000000] BIOS-provided physical RAM map:
> [    0.000000]  BIOS-e820: 0000000000000000 - 000000000009fc00 (usable)
> [    0.000000]  BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
> [    0.000000]  BIOS-e820: 00000000000e0000 - 0000000000100000 (reserved)
> [    0.000000]  BIOS-e820: 0000000000100000 - 000000003ed93000 (usable)
> [    0.000000]  BIOS-e820: 000000003ed93000 - 000000003ee4d000 (ACPI NVS)
> [    0.000000]  BIOS-e820: 000000003ee4d000 - 000000003fea2000 (usable)
> [    0.000000]  BIOS-e820: 000000003fea2000 - 000000003fee9000 (ACPI NVS)
> [    0.000000]  BIOS-e820: 000000003fee9000 - 000000003feed000 (usable)
> [    0.000000]  BIOS-e820: 000000003feed000 - 000000003feff000 (ACPI data)
> [    0.000000]  BIOS-e820: 000000003feff000 - 000000003ff00000 (usable)
>
> look at the RAM splitup:
>
>  640K + BIOS-hole + ~1GB + acpi + 17MB + acpi + 16K + acpi + 4K
>
> and the end of it is not 1024 MB but 1023 MB.

what were those BIOS engineer doing?

>
> so the _best_ mapping strategy would probably be to do 2MB granular
> mapping up to 1GB, i.e. to 'overmap' into the end of RAM. But we also
> have to make sure that we have no PCI resources or weird chipset
> resources in the final 1MB that could hurt us with PAT, aliasing-wise.
>
> Since i'm not sure we can really ensure sanity on that level, i guess
> your solution to precisely map everything without overmapping is our
> best choice. Thus sane hw with such end of RAM mappings:
>
>  BIOS-e820: 0000000100000000 - 0000000120000000 (usable)
>
> and another one with:
>
>  BIOS-e820: 0000000100000000 - 0000000830000000 (usable)
>
> ... would be slightly faster (because it would use 2MB TLBs at the end
> of kernel RAM, instead of broken-up 4K TLBs)
>
> perhaps we could also have a config and boot option that would sanitize
> the e820 map to just ignore all non-2MB granular RAM. Losing 1-2MB of
> RAM is not an issue on a 32GB system.

change left over to E820_RESERVED,

YH
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