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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

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.

Paolo


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