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





On 2015/9/2 19:09, Julien Grall wrote:
On 02/09/15 07:02, Shannon Zhao wrote:


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?

Hold on, this is about Linux able to use the memory for his own usage.
ACPI table are not part of this memory because they are marked reserved
by the firmware.

If we follow your logic, all ACPI tables always should be above the
kernel. I don't believe this is the case and it would be buggy on Xen
because of the DOM0 direct RAM mapping (i.e the first RAM bank can be
very high and the kernel too).


It looks weird. But from the booting.txt, it says the memory below base address is unusable and from early_init_dt_add_memory_arch in Linux, it really ignores the memblock below the PAGE_OFFSET.

I think the problem is how you reserved this region in the EFI memory
table. From what I saw, you marked this new memory with EFI_MEMORY_WB
(which means that the region can be usable by Linux).

Yes, I mark it with EFI_MEMORY_WB. Is this right?

--
Shannon

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