Re: fixed location of share info page in HVM guests

[Keir Fraser <keir.xen@gmail.com>]

On 28/08/2012 13:42, "Olaf Hering" <olaf@aepfle.de> wrote:

> On Tue, Aug 28, Keir Fraser wrote:
> 
>> Okay, that was a bit too clever, trying to hide between IOAPIC and LAPIC
>> pages. How about a bit lower in memory -- FE700000-FE7FFFFF?
>> 
>> Everything in range FC000000-FFFFFFFF should already be marked
>> E820_RESERVED. You can test that, and also see
>> tools/firmware/hvmloader/e820.c:build_e820_table() (and note that
>> RESERVED_MEMBASE == FC000000).
> 
> Yes, FC000000-FFFFFFFF has already an E820_RESERVED entry. Within that
> range the kernel finds the IOAPIC, LAPIC and the HPET, perhaps because
> they are listed in the ACPI table or because they are found by other
> ways.

Yes they are all listed in various ACPI tables.

> To make the location of the of the shared pages configurable from the
> tools, does tools/firmware/hvmloader/acpi/dsdt.asl have a way to
> describe such special region? Maybe the kernel parses that table early
> enough, before the shared_info page gets used.

If you have access to the ASL parser that early that would be awesome. You
can then just add a new name or something in the DSDT, like "Name (\XENR,
0xFE700000)" and then evaluate that Name object during boot to get the
numeric value (acpi_evaluate_object? Or acpi_evaluate_integer? Just
guessing!).

I have my doubts though... Some static ACPI tables are parsed very early,
but you need some more sophistication brought online before you can parse
out the DSDT and eval its contents. Well worth checking however, as sticking
this in the DSDT would be about the best option if it can work.

 -- Keir

> Olaf