[PATCH 3/5] arm64: efi: avoid block mappings for unaligned UEFI memory regions

[Ard Biesheuvel]

On 29 June 2016 at 19:00, Leif Lindholm <leif.lindholm at linaro.org> wrote:
> On Wed, Jun 29, 2016 at 06:53:18PM +0200, Ard Biesheuvel wrote:
>> On 29 June 2016 at 18:50, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
>> > On 29 June 2016 at 18:45, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> >> On Wed, Jun 29, 2016 at 02:51:28PM +0200, Ard Biesheuvel wrote:
>> >>> +     if (!PAGE_ALIGNED(md->phys_addr) ||
>> >>> +         !PAGE_ALIGNED(md->num_pages << EFI_PAGE_SHIFT)) {
>> >>> +             /*
>> >>> +              * If the end address of this region is not aligned to page
>> >>> +              * size, the mapping is rounded up, and may end up sharing a
>> >>> +              * page frame with the next UEFI memory region. If we create
>> >>> +              * a block entry now, we may need to split it again when mapping
>> >>> +              * the next region, and support for that is going to be removed
>> >>> +              * from the MMU routines. So avoid block mappings altogether in
>> >>> +              * that case.
>> >>> +              */
>> >>> +             allow_block_mappings = false;
>> >>> +     }
>> >>
>> >> How common is it for large areas to have unaligned start/end? I wonder
>> >> whether it's worth implementing my approach to look ahead and explicitly
>> >> check the overlap with the next section instead of disabling block
>> >> mappings altogether for this region.
>> >>
>> >
>> > Very uncommon. Typically, only MMIO regions that represent NOR flash
>> > are larger than a couple of pages. Taken from QEMU:
>>
>>   RT_Code   :            640 Pages (2,621,440 Bytes)
>>   RT_Data   :            880 Pages (3,604,480 Bytes)
>>
>> so all RT_Code regions *combined* are 2.5 MB in total, and all RT_Data
>> regions 3.5 MB. Ideally, they are grouped together, but in reality,
>> there are always a couple of regions of each type, so there is little
>> to gain here from using block mappings
>
> Is this representative for real platforms?
>

I think it is a reasonable ballpark figure

> What about efifb and reserved regions?
>

Those are not tagged as EFI_MEMORY_RUNTIME so they are not covered by
the UEFI runtime mappings, and not relevant to this discussion.

> My (x86) Lenovo workstation has one 64MB and one 16MB Runtime/Uncached
> MMIO region. As well as a 3MB and a 4MB RT_Data one.
>

Are those MMIO regions naturally aligned? And how about the RT_Data ones?