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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 18.12.12 at 14:28, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> Il 18/12/2012 14:27, Jan Beulich ha scritto:
>>>>> On 18.12.12 at 14:22, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>>> Il 18/12/2012 14:07, Jan Beulich ha scritto:
>>>>>>> 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?
>>>
>>> No, just busy.  I agree that checking all three is best.  However, there
>>> is at least one known case where 0x40e doesn't work, so 0x413 and
>>> multiboot should be enough.
>> 
>> Can you provide more detail about this specific case? In
>> particular, what value 0x40e in fact has there?
> 
> Sure.  0x40e did point to the beginning of the EBDA (around 635k), but
> an option ROM was reserving memory below there by lowering 0x413.
> That's the "on some machines" in the commit message.

That wouldn't preclude the suggested sanity-checked-minimum
solution.

Jan


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