Re: [Xen-devel] Supporting systems with large E820 maps

[Juergen Gross <jgross@xxxxxxxx>]

On 21/03/17 11:05, Jan Beulich wrote:
>>>> On 21.03.17 at 06:14, <jgross@xxxxxxxx> wrote:
>> On 20/03/17 20:03, Alex Thorlton wrote:
>>> Hey everyone,
>>>
>>> Recently, I've been working with Boris Ostrovsky to get Xen running on
>>> some of our larger systems, and we've run into a few problems with the
>>> amount of space that Xen sets aside for the E820 map.
>>>
>>> The first problem that I hit was that E820MAX is far too small, at 128
>>> entries, for the system that we're testing with.  The EFI memory map
>>> handed up from the boot loader tops out at 783 entries, which far
>>> exceeds the amount of space allocated for the memory map in
>>> arch/x86/boot/mem.S.  I was able to get past this problem by bumping
>>> E820MAX up to 1024 in arch/x86/boot/mem.S and include/asm-x86/e820.h.
>>>
>>> The second problem that I encountered was that Xen uses a signed char to
>>> store the number of entries in the memory map in a few places, which is
>>> too small to hold the number of entries after bumping E820MAX up to
>>> 1024.  I made the following changes to get past this:
>>
>> The problem with setting E820MAX to a higher value in mem.S without
>> further measures is that you are growing the trampoline size. This is
>> problematic for memory allocation in the multiboot path.
>>
>> I have some patches sitting here waiting for Daniel's multiboot series
>> to go in. My patches are not using the mem.S e820 array for the EFI
>> memory map, so the BIOS memory map buffer can remain smaller while the
>> EFI buffer can be made rather large. This avoids growing the trampoline
>> (in fact I've managed to reduce it to a single page).
>>
>> I didn't post my series up to now in order to not block Daniel's series
>> again. So what do people think: should I wait some more time with my
>> patches, or should I send them rather soon?
> 
> I'd say go ahead - whether the rest of Daniel's series will go in for
> 4.9 is undetermined at this point in time. At the very least we first
> need to get details on the boot regression Andrew says they're
> observing with what has gone in already.

Okay. I think I'll just post the first three patches which basically
will support more EFI memory map entries without affecting the
assembler parts too much.


Juergen


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