Re: [Xen-devel] [PATCH v11 06/13] efi: create new early memory allocator

[Julien Grall <julien.grall@xxxxxxx>]

Hi Daniel,

On 05/12/16 22:25, Daniel Kiper wrote:
> There is a problem with place_string() which is used as early memory
> allocator. It gets memory chunks starting from start symbol and goes
> down. Sadly this does not work when Xen is loaded using multiboot2
> protocol because then the start lives on 1 MiB address and we should
> not allocate a memory from below of it. So, I tried to use mem_lower
> address calculated by GRUB2. However, this solution 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... :-(((
> Hence, we need new memory allocator for Xen EFI boot code which is
> quite simple and generic and could be used by place_string() and
> efi_arch_allocate_mmap_buffer(). I think about following 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
>    it in e820 memory map and map it in Xen virtual address space. This
>    means that the code referring to Xen command line, loaded modules and
>    EFI memory map, mostly in __start_xen(), will be further complicated
>    and diverge from legacy BIOS cases. Additionally, both former things
>    have to be placed below 4 GiB because their addresses are stored in
>    multiboot_info_t structure which has 32-bit relevant members.
>
> 2) We may allocate memory area statically somewhere in Xen code which
>    could be used as memory pool for early dynamic allocations. Looks
>    quite simple. Additionally, it would not depend on EFI at all and
>    could be used on legacy BIOS platforms if we need it. However, we
>    must carefully choose size of this pool. We do not want increase Xen
>    binary size too much and waste too much memory but also we must fit
>    at least memory map on x86 EFI platforms. As I saw on small machine,
>    e.g. IBM System x3550 M2 with 8 GiB RAM, memory map may contain more
>    than 200 entries. Every entry on x86-64 platform is 40 bytes in size.
>    So, it means that we need more than 8 KiB for EFI memory map only.
>    Additionally, if we use this memory pool for Xen and modules command
>    line storage (it would be used when xen.efi is executed as EFI application)
>    then we should add, I think, about 1 KiB. In this case, to be on safe
>    side, we should assume at least 64 KiB pool for early memory allocations.
>    Which is about 4 times of our earlier calculations. However, during
>    discussion on Xen-devel Jan Beulich suggested that just in case we should
>    use 1 MiB memory pool like it is in original place_string() implementation.
>    So, let's use 1 MiB as it was proposed. If we think that we should not
>    waste unallocated memory in the pool on running system then we can mark
>    this region as __initdata and move all required data to dynamically
>    allocated places somewhere in __start_xen().
>
> 2a) We could put memory pool into .bss.page_aligned section. Then allocate
>     memory chunks starting from the lowest address. After init phase we can
>     free unused portion of the memory pool as in case of .init.text or 
> .init.data
>     sections. This way we do not need to allocate any space in image file and
>     freeing of unused area in the memory pool is very simple.
>
> Now #2a solution is implemented because it is quite simple and requires
> limited number of changes, especially in __start_xen().
>
> New allocator is quite generic and can be used on ARM platforms too.
> Though it is not enabled on ARM yet due to lack of some prereq.
> List of them is placed before ebmalloc code.
>
> Signed-off-by: Daniel Kiper <daniel.kiper@xxxxxxxxxx>
FWIW,

Acked-by: Julien Grall <julien.grall@xxxxxxx>

--
Julien Grall

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