[Xen-changelog] [xen stable-4.2] x86/boot: fix BIOS memory corruption on certain IBM systems

[patchbot@xxxxxxx]

commit 6bbf2a656a44cafe0294049aa4fd2559bd49c18d
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Dec 9 14:47:43 2013 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Dec 9 14:47:43 2013 +0100

    x86/boot: fix BIOS memory corruption on certain IBM systems
    
    IBM System x3530 M4 BIOSes (including the latest available at the time of 
this
    patch) will corrupt a byte at physical address 0x105ff1 to the value of 0x86
    if %esp has the value 0x00080000 when issuing an `int $0x15 (ax=0xec00)` to
    inform the system about our intended operating mode.
    
    Xen gets unhappy when the bootloader has placed it's .text section in over
    this specific region of RAM.
    
    After dropping into 16bit mode, clear all 32 bits of %esp, and for the BIOS
    call already documented to be affected by BIOS bugs clear all GPRs.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Acked-by: Keir Fraser <keir@xxxxxxx>
    Release-acked-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 1ed76797439e384de18fcd6810bd4743d4f38b1e
    master date: 2013-12-06 11:28:00 +0100
---
 xen/arch/x86/boot/trampoline.S |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index 4421fc2..9927b7a 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -165,7 +165,7 @@ trampoline_boot_cpu_entry:
         mov     %ax,%ss
 
         /* Initialise stack pointer and IDT, and enable irqs. */
-        xor     %sp,%sp
+        xor     %esp,%esp
         lidt    bootsym(rm_idt)
         sti
 
@@ -174,6 +174,11 @@ trampoline_boot_cpu_entry:
          * Declare that our target operating mode is long mode.
          * Initialise 32-bit registers since some buggy BIOSes depend on it.
          */
+        xor     %ecx,%ecx
+        xor     %edx,%edx
+        xor     %esi,%esi
+        xor     %edi,%edi
+        xor     %ebp,%ebp
         movl    $0xec00,%eax      # declare target operating mode
         movl    $0x0002,%ebx      # long mode
         int     $0x15
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.2

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