Hi Andrew, On 06/12/2017 11:45 AM, Andrew Cooper wrote:
c/s b28044226e1 "x86: make Xen early boot code relocatable" introducesmov $sym_offs(__image_base__),%esi to the legacy boot path. However, this is by definition 0, which means the boot code only functions correctly when Xen is loaded at its preferred physical address (2M at the time of writing). Xen does cope if loaded at an alternative physical address, if the MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR tag is filled in properly. While recent versions of Grub do fill this in appropriately, tboot does not. (In fact, tboot loads Xen at the preferred address, but claims a load address of 8M.) Both Multiboot 1 and 2 specify the execution environment as being flat. As a result, Xen needs no help calculating the proper load address. However, Multiboot specifies %esp as undefined. Experimentally, using the entry %esp is fine, but this is certainly no guarantee. Use a temporary stack in the first page of RAM, which is one of the safest areas to clobber. Calculate the load address from %eip alone, and ignore MULTIBOOT2_TAG_TYPE_LOAD_BASE_ADDR entirely. This fixes legacy boot under various versions of tboot. Finally, set up the stack as soon as possible, which means the BIOS path has a usable stack for the entirety of its duration. Use the full available stack size, rather than limiting to an arbitrary 1k. One side effect is that the MB2/EFI path continues to use the EFI stack until the trampoline is entered. Signed-off-by: Andrew Cooper <[email protected]> Tested-by: Sergey Dyasli <[email protected]> --- CC: Jan Beulich <[email protected]> CC: Julien Grall <[email protected]> CC: Daniel Kiper <[email protected]> CC: Doug Goldstein <[email protected]> CC: Sergey Dyasli <[email protected]> This is a regression introduced in Xen 4.9, and should therefore be fixed.
This is touching early-boot code. I would like to wait a least a push with this patch on staging before suggesting to push in Xen 4.9.
Cheers, -- Julien Grall _______________________________________________ Xen-devel mailing list [email protected] https://lists.xen.org/xen-devel
