[PATCH v2] x86: Use low memory size directly from Multiboot

[Tu Dinh Ngoc <dinhngoc.tu@xxxxxxx>]

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.

Use the information directly supplied by Multiboot boot information
instead.
---
 xen/arch/x86/boot/head.S | 34 ++++++++++++----------------------
 1 file changed, 12 insertions(+), 22 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index dd1bea0d10..62fe3fe55b 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -524,33 +524,23 @@ 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
-0:
-        movzwl  0x413,%ecx          /* use base memory size on failure */
-        shl     $10-4,%ecx
-1:
+        /* Use lower memory size directly from Multiboot */
+        mov     %edx,%ecx
         /*
-         * Compare the value in the BDA with the information from the
-         * multiboot structure (if available) and use the smallest.
+         * Old Kexec used to report the value in bytes instead of kilobytes
+         * like it's supposed to, so fix that if detected.
          */
-        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 */
+        cmpl    $640,%ecx
+        jbe     1f
+        shr     $10,%ecx
+1:
+        /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
+        shr     $2,%ecx
 
-2:
         /* Reserve memory for the trampoline and the low-memory stack. */
-        sub     $((TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE)>>4),%ecx
+        sub     $((TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE)>>12),%ecx
 
-        /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
-        xor     %cl, %cl
-        shl     $4, %ecx
+        shl     $12,%ecx
         mov     %ecx,sym_esi(trampoline_phys)
 
 trampoline_setup:
-- 
2.25.1