Re: Design doc of adding ACPI support for arm64 on Xen - version 5

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2015/9/1 19:28, Julien Grall wrote:
> Hi Shannon,
> On 01/09/15 05:12, Shannon Zhao wrote:
>> I tried this. Directly use the "kinfo->gnttab_start = __pa(_stext)" as
>> the address where these tables are mapped to Dom0. But the value of
>> gnttab_start is lower than the start of RAM, so Dom0 ingore these
>> regions and boot failed. see early_init_dt_add_memory_arch()
> 
> Can you elaborate? How Linux will fail? If this region is marked as
> reserved in the UEFI memory map, Linux will mark the memory as reserved.
> 
> Furthermore, *ioremap is used in order to map the EFI tables so I don't
> see a reason to fail.
> 

It's fine to parse EFI table but fails to parse ACPI table.

It doesn't add the memblock since it doesn't pass below check in
early_init_dt_add_memory_arch:
	if (base + size < phys_offset) {
		pr_warning("Ignoring memory block 0x%llx - 0x%llx\n",
			   base, base + size);
		return;
	}

It's due to kinfo->gnttab_start (e.g. 0x87e00000) lower than the memory
start address (e.g. 0x90000000).

Then Linux will fail at parsing ACPI table.

ACPI: Interpreter enabled
ACPI: Using GIC for interrupt routing
Unhandled fault: alignment fault (0x96000021) at 0xffffff8000068184
Internal error: : 96000021 [#1] PREEMPT SMP
Modules linked in:
CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.2.0-rc6+ #143
Hardware name: (null) (DT)
task: ffffffc008870000 ti: ffffffc00884c000 task.ti: ffffffc00884c000
PC is at acpi_get_phys_id+0x264/0x290
LR is at acpi_get_phys_id+0x178/0x290

>>>>
>>>> In addition, how does UEFI find the space to place the tables? Could we
>>>> use the same way?
>>>
>>> I think that those tables are living in the RAM and region used are
>>> marked as reserved.
>>>
>>
>> So can we use the same way for Dom0? I think the Linux will reserve the
>> regions for EFI in reserve_regions(). Therefore, Dom0 will not use these
>> reserved regions for other use.
> 
> Jan had some concerned about putting the EFI tables in RAM owned by DOM0
> (see [1]).
> 
> Can you explain how Linux behave with EFI tables. I.e:
> 	- Where tables are expected to live (RAM, others...)?
I'm not sure about this.

> 	- Are thoses regions freed at some point to be re-use?

I look at how Linux reserve these regions. See it in reserve_regions()
(arch/arm64/kernel/efi.c). It uses memblock_add() to add them to global
memblock and then uses memblock_reserve() to reserve them (like
allocating some memory) for certain use (here we use them to store the
tables). And I didn't see other places call memblock_free() to free them.

-- 
Shannon