Re: [Xen-devel] [PATCH v3 12/16 - RFC] x86/efi: create new early memory allocator

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 15.04.16 at 14:33, <daniel.kiper@xxxxxxxxxx> wrote:
> There is a problem with place_string() which is used as early memory
> allocator. It gets memory chunks starting from start symbol and
> going down. Sadly this does not work when Xen is loaded using multiboot2
> protocol because start lives on 1 MiB address. So, I tried to use
> mem_lower address calculated by GRUB2. However, it works only on some
> machines. There are machines in the wild (e.g. Dell PowerEdge R820)
> which uses first ~640 KiB for boot services code or data... :-(((
> 
> In case of multiboot2 protocol we need that place_string() only allocate
> memory chunk for EFI memory map. However, I think that it should be fixed
> instead of making another function used just in one case. I thought about
> two solutions.
> 
> 1) We could use native EFI allocation functions (e.g. AllocatePool()
>    or AllocatePages()) to get memory chunk. However, later (somewhere
>    in __start_xen()) we must copy its contents to safe place or reserve
>    this in e820 memory map and map it in Xen virtual address space.
>    In later case we must also care about conflicts with e.g. crash
>    kernel regions which could be quite difficult.

I don't see why that would be: Simply use an allocation type that
doesn't lead to the area getting consumed as normal RAM. Nor do
I see the kexec collision potential. Furthermore (and I think I've
said so before) ARM is already using AllocatePool() - just with an
unsuitable memory type -, so doing so on x86 too would allow for
efi_arch_allocate_mmap_buffer() to go away.

> Jan Beulich added 1b) Do away with efi_arch_allocate_mmap_buffer() and use
>    AllocatePages() uniformly, perhaps with a per-arch specified memory type
>    (by means of which you can control whether the memory contents will remain
>    preserved until the time you want to look at it). That will eliminate the
>    only place_string() you're concerned about, with a patch with better
>    diffstat (largely due to the questionable arch hook gone).
> 
> However, this solution does not solve conflicts problem described in #1
> because EFI memory map is needed during Xen runtime after init phase.
> So, finally we would get back to #1. Hmmm... Should I check how Linux
> and others cope with that problem?

Ah, here you mention it actually. Yet you don't explain what conflict
potential you see once using EfiRuntimeServicesData for the allocation.

Jan


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