Re: [PATCH -v2 0/4] EFI 1:1 mapping

["H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>]

If you cap it you are basically imposing a constraint on the firmware and may not run properly (or at least have to turn off EFI runtime calls with all that implies.)  It might be good to have a sanity check but it needs to be pretty generous.

Borislav Petkov <bp-Gina5bIWoIWzQB+pC5nmwQ@public.gmane.org> wrote:

>On Thu, Jun 20, 2013 at 03:35:24PM -0700, H. Peter Anvin wrote:
>> On 06/20/2013 11:47 AM, Borislav Petkov wrote:
>> > 
>> > I guess we can do a top-down allocation, starting from the highest
>> > virtual addresses:
>> > 
>> > EFI_HIGHEST_ADDRESS
>> > |
>> > | size1
>> > |
>> > --> region1
>> > |
>> > | size2
>> > |
>> > --> region2
>> > 
>> > ...
>> > 
>> > and we make EFI_HIGHEST_ADDRESS be the same absolute number on
>every
>> > system.
>> > 
>> > hpa, is this close to what you had in mind? It would be prudent to
>> > verify whether this will suit well with the kexec virtual space
>layout
>> > though...
>> > 
>> 
>> This would work really well, I think.  The tricky part here is to
>pick a
>> safe EFI_HIGHEST_ADDRESS as it is an ABI.
>> 
>> My preference would be to make EFI_HIGHEST_ADDRESS = -4 GB, which is
>> *not* what Windows uses, but will leave the high negative range
>clear,
>> and allows a range where we can grow down without much risk of
>> interfering with anything else.
>
>Hmm, cool. Let me see whether my primitive math still has it:
>
>-(4 << 30) = 0xffffffff00000000.
>
>Staring at Documentation/x86/x86_64/mm.txt, that's right in the unused
>hole, sandwiched between:
>
>ffffea0000000000 - ffffeaffffffffff (=40 bits) virtual memory map (1TB)
>
>xxxxxxxxxxxxxxxx - ffffffff00000000 (=XX bits, not a lot :-), maybe 4,
>i.e. 64G) EFI
>
>ffffffff80000000 - ffffffffa0000000 (=512 MB)  kernel text mapping,
>from phys 0
>
>Now, if we go and do that, what are we going to say for the lower
>bound,
>in case later someone wants to use some more of the rest of the unused
>hole? Should we limit it to say
>
>0xffffffff00000000 -
>0xfffffff000000000 = 64G max EFI mappable region.
>
>Or am I too generous? The remaining hole is around
>
>(0xfffffff000000000 - 0xffffeaffffffffff) >> 40 = 20TB.
>
>Thanks.

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