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.ki...@oracle.com>
FWIW, Acked-by: Julien Grall <julien.gr...@arm.com> -- Julien Grall _______________________________________________ Xen-devel mailing list Xen-devel@lists.xen.org https://lists.xen.org/xen-devel
