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

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2015/9/1 21:40, Julien Grall wrote:
> On 01/09/15 13:35, Shannon Zhao wrote:
>>
>>
>> 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
> 
> IIRC, this is because Linux will consider the region as non-RAM (see
> acpi_os_ioremap in arch/arm64/include/asm/acpi.h).
> 
> IHMO this is not a problem in the design but a bug in Linux/Xen.
> 
> You need to see how to make Linux see the region as a RAM (either by
> early_init_dt_add_memory_arch or memblock_reserve). This would mean
> either change the way you describe the region in the UEFI memory map or
> fix Linux.
> 
There are some descriptions in Documentation/arm64/booting.txt of Linux:

"The Image must be placed text_offset bytes from a 2MB aligned base
address near the start of usable system RAM and called there. Memory
below that base address is currently unusable by Linux, and therefore it
is strongly recommended that this location is the start of system RAM.
At least image_size bytes from the start of the image must be free for
use by the kernel."

>From this, it says "Memory below that base address is currently unusable
by Linux". So if we put these tables below Dom0 RAM address and even
describe these regions as RAM, the Linux could not use them.

Any thoughts about this?

Thanks,
-- 
Shannon