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    Subject[tip:x86/trampoline] x86, realmode: Remove indirect jumps in trampoline_32 and wakeup_asm
    Commit-ID:  968ff9ee56f1e3ed4ff4a6d10185865dc77d8f7e
    Gitweb: http://git.kernel.org/tip/968ff9ee56f1e3ed4ff4a6d10185865dc77d8f7e
    Author: H. Peter Anvin <hpa@linux.intel.com>
    AuthorDate: Tue, 8 May 2012 21:22:36 +0300
    Committer: H. Peter Anvin <hpa@linux.intel.com>
    CommitDate: Tue, 8 May 2012 11:48:03 -0700

    x86, realmode: Remove indirect jumps in trampoline_32 and wakeup_asm

    Remove indirect jumps in trampoline_32.S and the 32-bit part of
    wakeup_asm.S. There exist systems which are known to do weird
    things if an SMI comes in right after a mode switch, and the
    safest way to deal with it is to always follow with a simple
    absolute far jump. In the 64-bit code we then to a register
    indirect near jump; follow that pattern for the 32-bit code.

    Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
    Link: http://lkml.kernel.org/r/1336501366-28617-14-git-send-email-jarkko.sakkinen@intel.com
    ---
    arch/x86/realmode/rm/trampoline_32.S | 22 +++++++++++++---------
    arch/x86/realmode/rm/wakeup/wakeup_asm.S | 8 +++++---
    2 files changed, 18 insertions(+), 12 deletions(-)

    diff --git a/arch/x86/realmode/rm/trampoline_32.S b/arch/x86/realmode/rm/trampoline_32.S
    index 1f9e331..1315ef4 100644
    --- a/arch/x86/realmode/rm/trampoline_32.S
    +++ b/arch/x86/realmode/rm/trampoline_32.S
    @@ -47,24 +47,29 @@ trampoline_data:

    cli # We should be safe anyway

    + movl startup_32_smp, %eax # where we need to go
    +
    movl $0xA5A5A5A5, trampoline_status
    # write marker for master knows we're running

    - /* GDT tables in non default location kernel can be beyond 16MB and
    + /*
    + * GDT tables in non default location kernel can be beyond 16MB and
    * lgdt will not be able to load the address as in real mode default
    * operand size is 16bit. Use lgdtl instead to force operand size
    * to 32 bit.
    */
    -
    lidtl boot_idt_descr # load idt with 0, 0
    lgdtl boot_gdt_descr # load gdt with whatever is appropriate

    - xor %ax, %ax
    - inc %ax # protected mode (PE) bit
    - lmsw %ax # into protected mode
    + movw $1, %dx # protected mode (PE) bit
    + lmsw %dx # into protected mode

    - # flush prefetch and jump to startup_32_smp in arch/i386/kernel/head.S
    - ljmpl *(startup_32_smp)
    + ljmpl $__BOOT_CS, $pa_startup_32
    +
    + .section ".text32","ax"
    + .code32
    +ENTRY(startup_32) # note: also used from wakeup_asm.S
    + jmp *%eax

    .data
    .globl startup_32_smp, boot_gdt, trampoline_status
    @@ -82,5 +87,4 @@ trampoline_status:
    .long 0

    startup_32_smp:
    - .long 0x00000000
    - .word __BOOT_CS, 0
    + .long 0
    diff --git a/arch/x86/realmode/rm/wakeup/wakeup_asm.S b/arch/x86/realmode/rm/wakeup/wakeup_asm.S
    index b61126c..4c5c5f2 100644
    --- a/arch/x86/realmode/rm/wakeup/wakeup_asm.S
    +++ b/arch/x86/realmode/rm/wakeup/wakeup_asm.S
    @@ -124,9 +124,11 @@ wakeup_start:
    lgdtl pmode_gdt

    /* This really couldn't... */
    - movl pmode_cr0, %eax
    - movl %eax, %cr0
    - ljmpl *pmode_entry
    + movl pmode_entry, %eax
    + movl pmode_cr0, %ecx
    + movl %ecx, %cr0
    + ljmpl $__KERNEL_CS, $pa_startup_32
    + /* -> jmp *%eax in trampoline_32.S */
    #else
    jmp trampoline_data
    #endif

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