Re: [Qemu-devel] [PATCH v2] hw/arm/virt-acpi - reserve ECAM space as PNP0C02 device

[Laszlo Ersek <lersek@redhat.com>]

On 01/18/17 16:18, Igor Mammedov wrote:
> On Tue, 17 Jan 2017 10:56:53 +0000
> Peter Maydell <peter.maydell@linaro.org> wrote:
> 
>> On 17 January 2017 at 09:49, Andrew Jones <drjones@redhat.com> wrote:
>>> In some cases the problem we're solving with the compat guards is
>>> a bit hypothetical, but, IMHO, nonetheless a good practice. While
>>> we may be sure that AAVMF and Linux will be fine with this table
>>> changing under their feet, we can't be sure there aren't other
>>> mach-virt users that have more sensitive firmwares/OSes. An ACPI-
>>> sensitive OS may notice the change on its next reboot after a
>>> migration, and then simply refuse to continue.  
>>
>> There's also the case where you do a VM migration midway through
>> UEFI booting up, I think, which might cause things to go wrong
>> if you catch it just at the wrong moment.
> acpi blobs are migrated from source so above won't happen.
> The time guest will see new table is fresh boot or reboot.
> 
>>
>>> Now, that said, I just spoke with Igor in order to learn the x86
>>> practice. He says that the policy has been more lax than what I
>>> suggest above. Hypothetical, low-risk issues are left unguarded,
>>> and only when a bug is found during testing is it then managed.
>>> The idea is to try and reduce the amount of compat variables and
>>> conditions needed in the ACPI generation code, but, of course, at
>>> some level of risk to users expecting their versioned machine type
>>> to always appear the same.
>>>
>>> So far we've been strict with mach-virt, guarding all hypothetical
>>> issues. Perhaps this patch is a good example to get a discussion
>>> started on whether or not we should be so strict though.  
>>
>> That said, I don't have a very strong opinion here, beyond that
>> we should be consistent at least with x86 practice.
> another reason why we are trying not to use strict approach with ACPI
> tables is that it's part of firmware and we didn't version firmwares
> so far. (i.e. dst host with newer QEMU will typically have newer
> firmware and guest with old machine-type migrated to host with newer
> QEMU will run new firmware on (re)boot)

I haven't been aware of this argument, and I'm surprised by it, but I
think it's valid. Regardless of our choice to ultimately compose the
ACPI tables in QEMU, guest OSes definitely consider ACPI as part of the
firmware. So, different ACPI content after a migration + guest reboot on
the target host is not much different from any other firmware-level
changes encountered on the same target host, after reboot.

Laszlo