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

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Dec 09, 2016 at 06:03:06PM +0000, Julien Grall wrote:
> 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>

Thanks a lot!

Daniel

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