Re: [Xen-devel] [PATCH] xen: find a better location for the real-mode trampoline

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 30.11.12 at 09:33, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>> On 29.11.12 at 18:34, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> On some machines, the location at 0x40e does not point to the beginning
>> of the EBDA.  Rather, it points to the beginning of the BIOS-reserved
>> area of the EBDA, while the option ROMs place their data below that
>> segment.
>> 
>> For this reason, 0x413 is actually a better source than 0x40e to get
>> the location of the real-mode trampoline.  But it is even better to
>> fetch the information from the multiboot structure, where the boot
>> loader has placed the data for us already.
> 
> I think if anything we really should make this a minimum calculation
> of all three (sanity checked) values, rather than throwing the other
> sources out. It's just not certain enough that we can trust all
> multiboot implementations.

I never saw a response from you on this one - were you
intending to follow up, or did you (silently) expect us to sort
this out?

Jan

> Of course, ideally we'd consult the memory map, but the E820 one
> is unavailable at that point (and getting at it would create a
> chicken-and-egg problem).
> 
> Jan
> 
>> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>> ---
>>  xen/arch/x86/boot/head.S | 21 ++++++++++++---------
>>  1 file changed, 12 insertions(+), 9 deletions(-)
>> 
>> diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
>> index 7efa155..1790462 100644
>> --- a/xen/arch/x86/boot/head.S
>> +++ b/xen/arch/x86/boot/head.S
>> @@ -78,16 +78,19 @@ __start:
>>          cmp     $0x2BADB002,%eax
>>          jne     not_multiboot
>>  
>> -        /* Set up trampoline segment 64k below EBDA */
>> -        movzwl  0x40e,%eax          /* EBDA segment */
>> -        cmp     $0xa000,%eax        /* sanity check (high) */
>> -        jae     0f
>> -        cmp     $0x4000,%eax        /* sanity check (low) */
>> -        jae     1f
>> -0:
>> -        movzwl  0x413,%eax          /* use base memory size on failure */
>> -        shl     $10-4,%eax
>> +        /* Set up trampoline segment just below end of base memory.
>> +         * Prefer to get this information from the multiboot
>> +         * structure, if available.
>> +         */
>> +        mov     4(%ebx),%eax        /* kb of low memory */
>> +        testb   $1,(%ebx)           /* test MBI_MEMLIMITS */
>> +        jnz     1f
>> +
>> +        movzwl  0x413,%eax          /* base memory size in kb */
>>  1:
>> +        shl     $10-4,%eax          /* convert to a segment number */
>> +
>> +        /* Reserve 64kb for the trampoline */
>>          sub     $0x1000,%eax
>>  
>>          /* From arch/x86/smpboot.c: start_eip had better be page-aligned! */
>> -- 
>> 1.8.0
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxx 
>> http://lists.xen.org/xen-devel 
> 
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxx 
> http://lists.xen.org/xen-devel 




_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel