Re: [Xen-devel] [PATCH RESEND 03/14] libxc: Add placeholders for ACPI tables blob and size

[Shannon Zhao <zhaoshenglong@xxxxxxxxxx>]

On 2016/6/16 18:21, Julien Grall wrote:
> 
> 
> On 16/06/16 07:53, Shannon Zhao wrote:
>> Hi Julien,
> 
> Hi Shannon,
> 
>>
>> On 2016/6/7 21:06, Julien Grall wrote:
>>>> That's its own mechanism I think and UEFI wants all the memory maps
>>>> under its control. And it's same for QEMU(x86 and ARM) and also for Xen
>>>> on x86. You can have a look at the OvmfPkg/AcpiPlatformDxe/Xen.c which
>>>> is used for x86 Xen DomU.
>>>
>>> UEFI cannot control the memory map because it is not capable to know
>>> what a region is used for (such as magic pages, grant table...).
>>>
>>> In the case of domU for x86, the ACPI table are located in predefine
>>> physical address by hvmloader.
>> The truth is that hvmloader puts the tables at address
>> ACPI_PHYSICAL_ADDRESS(0x000EA020), but UEFI will install the tables and
>> relocate them as well. You can see OvmfPkg/AcpiPlatformDxe/Xen.c in edk2
>> source code.
>>
>> So I think it's same with ARM except x86 puts the tables at one fixed
>> address while ARM dynamically computes the address and passes the
>> address information to UEFI through dts.
>>
>> Yeah, of course we can let ARM put the tables at one fixed address and
>> also it doesn't need the ACPI module to pass the address and let UEFI
>> find the RSDP table from the fixed address. But what's the difference
>> between the fixed and dynamicall ones? Because both ways UEFI will
>> install and relocate the tables.
> 
> You seem to think that OVMF is the only UEFI implementation available
> and its behavior is set in stone. Can you quote the UEFI spec stating
> the ACPI tables will always be relocated?
> 
> If not, putting the ACPI module right in the middle of memory is not the
> wisest choice because the tables can not be easily relocated like other
> modules (DT, Kernel, initramfs).
> 
The ACPI tables are put after the DT. How could you say the ACPI is in
the middle while DT not?

> My suggestion to put the ACPI tables at a static address outside the RAM
> is to let the choice to the firmware to do whatever it wants without
> impacting the performance of a kernel if it ever decides to keep in
> place the tables.
> 
> However, this static address is from the toolstack point of view. The
> firmware should not assume any static address because the guest memory
> layout is not part of the ABI for Xen ARM (i.e it can be modified
> between two releases). This is to allow us reshuffling the layout to
> make space for new features.
> 
Sure? Currently for Xen x86, edk2 uses XEN_ACPI_PHYSICAL_ADDRESS to find
RSDP. If we put the tables at another address, DomU will fail to boot.

>>
>>> I really don't see any reason that would
>>> prevent us to do the same on ARM.
>>>
>>> If the UEFI firmware wants to relocate the tables, then fine. But we
>>> should also think about any firmware which may not relocate the tables.
>> Can you name one firmware except the UEFI?
> 
> Sorry I meant any other UEFI implementation (i.e other than OVMF) may
> not relocate the tables.
So please tell me the name of other UEFI implementation.

Thanks,
-- 
Shannon


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