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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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