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).
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.
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.
Cheers,
--
Julien Grall
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