Re: [PATCH V2] xen/arm: Initialize acpi_disabled to true during declaration

[Oleksandr Tyshchenko <Oleksandr_Tyshchenko@xxxxxxxx>]

On 25.03.25 18:09, Julien Grall wrote:


> Hi Oleksandr,

Hello Julien

> 
> On 25/03/2025 16:05, Oleksandr Tyshchenko wrote:
>>>>> Furthermore, what happen if we decide to use ACPI afterwards? Wouldn't
>>>>> this mean that the static regions would be reserved even if ACPI 
>>>>> doesn't
>>>>> use static memory (all the memory is expected to be given to the
>>>>> allocator)?
>>>> I don't think such hybrid configuration is valid (booting with ACPI yet
>>>> declaring reserved regions in DT). See commit:
>>>> 9c2bc0f24b2ba7082df408b3c33ec9a86bf20cf0
>>>
>>> I don't think the commit is preventing hybrid configuration. It is just
>>> saying that the region (which could be a static region because this is
>>> not supported) will be unreserved.
>>>
>>> IOW, when booting with Device-Tree you may be able to use static memory.
>>> But if you were booting with ACPI, static memory is not supported and
>>> therefore the regions should be free for other purpose.
>>
>>
>> Julien, I see your points, but the current patch does not attempt to
>> make static (reserved) memory properly work on ACPI-based system (if it
>> is available there), current patch tries to solve the real issue on
>> device-tree-based system with Xen compiled with CONFIG_ACPI=y (at least
>> unintentionally). 
> 
> I am not asking to make ACPI work with static memory. I am asking to not 
> break ACPI if the Device-Tree is specifying static memory region.
> 
>> However, I wonder, why it has not been noticed so far.
> 
> ACPI is not a supported feature and gated by UNSUPPORTED. So the 
> implication is you have enabled UNSUPPORTED and anything can happen 
> really ;).

Indeed, this answers the question.


> 
>>
>> It took some time to understand why just enabling CONFIG_STATIC_MEMORY=y
>> triggered a BUG in common code. And it turned out that it was
>> CONFIG_ACPI=y in my Xen's .config that caused that consequence (I
>> specially wrote so long description to provide full context).
> 
> As I wrote above, the only thing I am asking is that memory for static 
> regions should be unreserved when ACPI is enabled like it is already the 
> case today.

So the concern is that not reserving provided by the device tree static 
memory region is going to be a potential waste of the memory on the real 
ACPI system? Or I missed something?

I see two options with unreserving the static memory regions on the real 
ACPI system. I assume, that we should unreserve only after we definitely 
know that we are running with ACPI (i.e. after acpi_boot_table_init() 
completes and acpi_disabled reflects the real state of things), right?

1. either call acpi_boot_table_init() before setup_mm() in Arm64's 
start_xen().
2. or go through reserved_mem->nr_banks and unreserve everything marked 
with MEMBANK_STATIC_DOMAIN after acpi_boot_table_init() completes

What do you think?

Unfortunately, I do not have such environment in hand (ACPI-based 
(dom0less?) system but still providing device-tree with static memory) 
for the real testing.

> 
> Cheers,
>