Previously, Xen used information from the BDA to detect the amount of available low memory. This does not work on some scenarios such as Coreboot, or when booting from Kexec on a UEFI system without CSM.
Prioritize the information supplied by Multiboot instead. If this is not available, fall back to the old BDA method. Signed-off-by: Tu Dinh Ngoc <[email protected]> --- Changes in v3: - Prioritize using Multiboot's memory information.. Fall back to using BDA in case MBI does not supply memory info. Changes in v2: - Detect if Multiboot claims there's more than 640 KB of low memory (happens with old Kexec versions), and correct the memory unit in such cases. --- xen/arch/x86/boot/head.S | 44 ++++++++++++++++++++++++++-------------- 1 file changed, 29 insertions(+), 15 deletions(-) diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S index dd1bea0d10..da7810060e 100644 --- a/xen/arch/x86/boot/head.S +++ b/xen/arch/x86/boot/head.S @@ -524,27 +524,41 @@ trampoline_bios_setup: mov %ecx,%fs mov %ecx,%gs - /* Set up trampoline segment 64k below EBDA */ - movzwl 0x40e,%ecx /* EBDA segment */ - cmp $0xa000,%ecx /* sanity check (high) */ - jae 0f - cmp $0x4000,%ecx /* sanity check (low) */ - jae 1f + /* Check if Multiboot provides us with low memory size. */ + mov %edx,%ecx + test %ecx,%ecx + jz 1f + + /* + * Old Kexec used to report memory sizes in bytes instead of kilobytes + * like it's supposed to. + * + * If Multiboot reports more than 640 KB of low memory, assume we have + * this problem. + */ + cmp $640,%ecx + jbe 0f + shr $10,%ecx 0: - movzwl 0x413,%ecx /* use base memory size on failure */ + /* %ecx now contains the low memory size in kilobytes. */ shl $10-4,%ecx + jmp 3f + 1: /* - * Compare the value in the BDA with the information from the - * multiboot structure (if available) and use the smallest. + * Multiboot doesn't provide us with memory info. Set up trampoline + * segment 64k below EBDA as fallback. */ - cmp $0x100,%edx /* is the multiboot value too small? */ - jb 2f /* if so, do not use it */ - shl $10-4,%edx - cmp %ecx,%edx /* compare with BDA value */ - cmovb %edx,%ecx /* and use the smaller */ - + movzwl 0x40e,%ecx /* EBDA segment */ + cmp $0xa000,%ecx /* sanity check (high) */ + jae 2f + cmp $0x4000,%ecx /* sanity check (low) */ + jae 3f 2: + movzwl 0x413,%ecx /* use base memory size on failure */ + shl $10-4,%ecx + +3: /* Reserve memory for the trampoline and the low-memory stack. */ sub $((TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE)>>4),%ecx -- 2.25.1
