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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.12.12 at 22:23, Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx> wrote:
> On Fri, Nov 30, 2012 at 08:33:34AM +0000, Jan Beulich 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.
>> 
>> 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).
> 
> Can we scan the memory for the possible EBDA regions? There is an
> "EBDA" type header in those regions, if I recall?

I don't think there are any signatures - the value at (real mode)
0040:000e has to be relied upon.

Jan


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