Re: [Xen-devel] [PATCH 1 of 3] Enable UEFI BIOS(OVMF) support in Xen-unstable HVM

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 22/07/2011 19:38, "Andrei Warkentin" <andreiw@xxxxxxxxxxxx> wrote:

> On Fri, Jul 22, 2011 at 11:38 AM, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
> 
>> Looks pretty decent. I wonder why you need to change get_shared_info() --
>> the existing mapping location is unused at the time hvmloader runs, and you
>> instead map it over the top of a page of RAM. If you want shared_info mapped
>> elsewhere, you can map it wherever you like as soon as your BIOS payload
>> takes over.
>> 
> 
> The problem is that this page lies in an unsafe for OVMF area (right
> below 4GB). In a typical PC environment,
> you have the firmware ROM decoding the physical address space right
> below 4GB, and it also has a chunk (~64-128k) shadowed below 1MB for
> legacy reasons. The EFI firmware bootstrap code is written with the
> assumption that it can transfer control to code < 4GB that will
> finalize the 16->PM-(>LM if 64) transitions and call C code. The
> get_shared_info page overlaps code we copy up below 4GB. This is why
> it was moved to a safer region.

Okay, we can work with this easily enough.

 -- Keir



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