Re: [Qemu-devel] How to reserve guest physical region for ACPI

[Laszlo Ersek <lersek@redhat.com>]

On 01/07/16 11:54, Michael S. Tsirkin wrote:
> On Thu, Jan 07, 2016 at 11:30:25AM +0100, Igor Mammedov wrote:
>> On Tue, 5 Jan 2016 18:43:02 +0200
>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>
>>> On Tue, Jan 05, 2016 at 05:30:25PM +0100, Igor Mammedov wrote:

...

>>>>>>> An alternative is to add an XSDT, XP ignores that.
>>>>>>> XSDT at the moment breaks OVMF (because it loads both
>>>>>>> the RSDT and the XSDT, which is wrong), but I think
>>>>>>> Laszlo was working on a fix for that.    
>>>>>> Using XSDT would increase ACPI tables occupied RAM
>>>>>> as it would duplicate DSDT + non XP supported AML
>>>>>> at global namespace.    
>>>>>
>>>>> Not at all - I posted patches linking to same
>>>>> tables from both RSDT and XSDT at some point.
>>>>> Only the list of pointers would be different.  
>>>> if you put XP incompatible AML in separate SSDT and link it
>>>> only from XSDT than that would work but if incompatibility
>>>> is in DSDT, one would have to provide compat DSDT for RSDT
>>>> an incompat DSDT for XSDT.  
>>>
>>> So don't do this.
>> well spec says "An ACPI-compatible OS must use the XSDT if present",
>> which I read as tables pointed by RSDT MUST be pointed by XSDT
>> as well and RSDT MUST NOT not be used.
>>
>> so if we put incompatible changes in a separate SSDT and put
>> it only in XSDT that might work. Showstopper here is OVMF which
>> has issues with it as Laszlo pointed out.
> 
> But that's just a bug.

Yes, but the bug (actually: lack of feature) is in the UEFI
specification. The current EFI_ACPI_TABLE_PROTOCOL implementation in
edk2 conforms to the specification. In order to expose the functionality
that the above trick needs, the UEFI spec has to be changed. In my
(limited, admittedly) experience, that's an uphill battle.

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