Subject: [PATCH 1/4] Documentation: arm: [U]EFI runtime services

From: [email-masked] (H. Peter Anvin)
Date: Thu, 27 Jun 2013 11:32:49 -0700

On 06/26/2013 07:38 AM, James Bottomley wrote:
> On Wed, 2013-06-26 at 14:59 +0100, Matt Fleming wrote:
>> On Wed, 26 Jun, at 03:53:11PM, Leif Lindholm wrote:
>>> It's completely feasible, but we'd need to use a different method to do
>>> the boot services call with a 1:1 mapping (idmap support is not available
>>> until much later in the boot process).
>>
>> At least if you no longer relied upon the idmap we could potentially
>> have a single efi_enter_virtual_mode() call-site in init/main.c, which
>> would be nice.
> 
> The fixed virtual address scheme currently being looked at for x86_64 to
> make SetVirtualAddressMap() kexec invariant doesn't work on 32 bit
> because the address space isn't big enough.  For ARM, given that we've
> much more opportunity to work with the vendors, can we just avoid
> transitioning to a virtual address map and always just install a
> physical mapping before doing efi calls?
> 

What we could do on x86-32 is to map from 0xc0000000 downwards.  It
wouldn't be invariant across kernel builds with different user/kernel
split... but I'm not sure we can win that one.

The other option is to say sod it and just use straight 1:1 mapping on
32 bits...

	-hpa