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Date: Tue, 9 Jun 2015 11:49:20 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 0/4]  acpi: xsdt support
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: ghammer@redhat.com, imammedo@redhat.com, lersek@redhat.com, qemu-devel@nongnu.org, shannon.zhao@linaro.org

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.


-- 
MST