Re: [Xen-devel] [PATCH] x86/boot: Fix BIOS memory corruption on certain IBM systems

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.12.13 at 21:34, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> --- a/xen/arch/x86/boot/trampoline.S
> +++ b/xen/arch/x86/boot/trampoline.S
> @@ -140,10 +140,12 @@ trampoline_boot_cpu_entry:
>  1:      mov     %cs,%ax
>          mov     %ax,%ds
>          mov     %ax,%es
> +        mov     %ax,%fs
> +        mov     %ax,%gs
>          mov     %ax,%ss
>  
>          /* Initialise stack pointer and IDT, and enable irqs. */
> -        xor     %sp,%sp
> +        xor     %esp,%esp

According to your findings this one line change is really all that's
needed. While I may be willing to accept the setting of %fs and
%gs, despite them being set to BOOT_PSEUDORM_DS right
before leaving protected mode (albeit I think it would be better
to clear them than to make them match %cs), ...

> @@ -151,6 +153,11 @@ 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

... I can't really see the value of the change here: If we're to
work around theoretical BIOS bugs, we'd need to do this prior to
each BIOS call. That's surely overkill. Therefore let's focus on
what is needed to work around _known_ BIOS bugs.

Jan


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