Re: [Qemu-devel] [PATCH v2 0/4] acpi: xsdt support

[Laszlo Ersek <lersek@redhat.com>]

On 08/27/15 19:58, Michael S. Tsirkin wrote:
> On Thu, Aug 27, 2015 at 04:27:00PM +0200, Laszlo Ersek wrote:
>> On 06/09/15 16:05, Michael S. Tsirkin wrote:
>>> On Tue, Jun 09, 2015 at 04:02:08PM +0200, Laszlo Ersek wrote:
>>>> On 06/09/15 11:49, Michael S. Tsirkin wrote:
>>>>> On Tue, Jun 09, 2015 at 02:02:31AM -0400, Paolo Bonzini wrote:
>>>>>> I would just patch OVMF to ignore the RSDT if there is an XSDT.
>>>>>>
>>>>>> Alternatively, can you check for ACPI 2.0 support via _OSI, and load the ACPI 2.0 bits via LoadTable? Hopefully XP does not BSOD if the invalid (for ACPI 1.0) opcodes are in a Then block or in a separate method... Then you can use just an RSDT.
>>>>>>
>>>>>> Paolo
>>>>>
>>>>>
>>>>> So it's even easier. The following doesn't crash XP:
>>>>>
>>>>>             Method(ADDR, 0, Serialized) {
>>>>> 		/* Region is local to method to avoid crashing ACPI 1.0 guests */
>>>>>                 DataTableRegion(IDPT, "UEFI", "BXPC", "VMGENI");
>>>>> 	        Field (IDPT, AnyAcc, NoLock, Preserve) {
>>>>> 		  Offset (54),
>>>>> 		  VGIA, 64     // address of "etc/acpi/vmgenid" blob
>>>>> 	        }
>>>>> 		Return(Add(VGIA), 40);
>>>>>             }
>>>>>
>>>>> Simply because XP doesn't ever call ADDR.
>>>>> Method must be serialized since attempts to create two
>>>>> regions or fields with the same name would crash OSPM.
>>>>
>>>> That sounds like a huge relief to me, so thank you for researching it.
>>>>
>>>> Laszlo
>>>
>>> Mind you, I still think it's worth it to support XSDT eventually,
>>> but the immediate need to do this in 2.4 timeframe is gone.
>>
>> ... I'll have to make the OVMF linker/loader a little bit smarter, so
>> that this QEMU patch series doesn't break it. Because, unfortunately,
>> XSDT seems to be a hard requirement for DataTableRegion(), according to
>> the ACPI spec. (Thus, unless SeaBIOS installs an XSDT too,
>> DataTableRegion() may not work in the ADDR method of the vmgenid device.)
> 
> That's not really true. ACPI is simply pushing XSDT everywhere,
> you will find it hardly ever mentions legacy tables like RSDP anymore.
> 
> And in practice, this works fine with all existing guests.
> 
> 
> So yes let's do it, but no, don't make this a dependency of
> this series.

Okay, thanks. I made a note for later.
Laszlo

>> (FWIW I checked the ACPICA source, and AFAICS, DataTableRegion() *of
>> ACPICA* should locate tables from the RSDT too just fine. But, IIRC,
>> Windows does not use ACPICA, and the ACPI spec requires XSDT. Quite
>> unfortunate.)
>>
>> The idea I have for OVMF is to track the offsets for each blob from
>> which tables have been installed already. Then I can detect further
>> pointers (located in any other blobs) pointing to the same offsets, and
>> forego duplicate table installations. This should fix the problem I
>> reported in this thread, and then this patch set should work for OVMF too.
>>
>> ... I've come across this today while writing "docs/specs/vmgenid.txt".
>>
>> Laszlo