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

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

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.

Paolo


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