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

[Ian Campbell <ian.campbell@citrix.com>]

On Wed, 2015-09-02 at 14:48 +0100, Julien Grall wrote:
> On 02/09/15 14:26, Ian Campbell wrote:
> > > > > 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?
> > > 
> > > I would say no, but it's only because I looked at the kernel code 
> > > quickly.
> > > 
> > > You have to looks how ACPI region/UEFI tables are described in the 
> > > host
> > > EFI memory map and mimicking for the DOM0 EFI memory map.
> > 
> > Surely it is the type (EfiACPIReclaimMemory, EfiACPIMemoryNVS etc) and 
> > not
> > the mapping attributes which should control whether an OS considers a
> > region usable? At least until the OS is done parsing tables neither of
> > those are usable (which implies we want NVS as our type, unless the 
> > memory
> > is intended to be reclaimed by dom0, implying it should own it).
> 
> It looks like that Linux on ARM64 is considering any region with
> EFI_MEMORY_WB set as normal RAM and will try to add as System RAM (see
> reserve_regions in arch/arm64/kernel/efi.c).

It's hard to believe this isn't a bug... It's probably worth asking the
Linux maintainers about this.

On first glance it seems to me that the is_reserve_region check ought to be
before the is_normal_ram one, not vice versa. But what do I know...

> At the same time, having WB set for a region that should be read-only
> looks like wrong to me.

it does seem a bit meaningless at least, if not wrong.

> 
> Regards,
>