Re: [Qemu-devel] [Linaro-acpi] [RFC PATCH 0/7] hw/arm/virt: Dynamic ACPI v5.1 table generation

[Hanjun Guo <hanjun.guo@linaro.org>]

On 2014-10-31 2:02, Mark Rutland wrote:
> On Thu, Oct 30, 2014 at 05:52:44PM +0000, Peter Maydell wrote:
>> On 30 October 2014 17:43, Alexander Spyridakis
>> <a.spyridakis@virtualopensystems.com> wrote:
>>> Currently, the virt machine model generates Device Tree information dynamically based on the existing devices in the system. This patch series extends the same concept but for ACPI information instead. A total of seven tables have been
>>> implemented in this patch series, which is the minimum for a basic ARM support.
>>>
>>> The set of generated tables are:
>>> - RSDP
>>> - XSDT
>>> - MADT
>>> - GTDT
>>> - FADT
>>> - FACS
>>> - DSDT
>>>
>>> The tables are created in standalone buffers, taking into account the
>>> needed information passed from the virt machine model. When the generation
>>> is finalized, the individual buffers are compacted to a single ACPI binary
>>> blob, where it is injected on the guest memory space in a fixed location.
>>> The guest kernel can find the ACPI tables by providing to it the physical
>>> address of the ACPI blob (e.g. acpi_rsdp=0x47000000 boot argument).
>>
>> (Sorry, I should have waited for the cover letter to arrive before replying.)
>>
>> I think this is definitely the wrong approach. We already have to
>> generate device tree information for the hardware we have, and having
>> an equivalent parallel infrastructure for generating ACPI as well
>> seems like it would be a tremendous mess. We should support guests
>> that require ACPI by having QEMU boot a UEFI bios blob and have that
>> UEFI code generate ACPI tables based on the DTB we hand it.
>> (Chances seem good that any guest that wants ACPI is going to want
>> UEFI runtime services anyway.)
> 
> Depending on why people want ACPI in a guest environment, generating
> ACPI tables from a DTB might not be possible (e.g. if they want to use
> AML for some reason).

Agreed.

> 
> So the important question is _why_ the guest needs to see an ACPI
> environment. What exactly can ACPI provide to the guest that DT does not
> already provide, and why is that necessary? What infrastrucutre is
> needed for that use case?

There is important feature called system device dynamic reconfiguration,
you know, hot-add/remove, if a gust need more/less memory or CPU, can we
add or remove them dynamically with DT? ACPI can do this, but I have no
idea if DT can. (Sorry, I don't know much about DT)

Thanks
Hanjun