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

[Leif Lindholm]

On Wed, Jun 29, 2016 at 07:04:57PM +0200, Ard Biesheuvel wrote:
> 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.

OK.

> > 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?

So, I've now gone home and don't have access to the Lenovo, however I
have a machine at home also with an AMI UEFI implementation, and
identical MMIO regions. And they do look naturally aligned.

The RT_Data ones are not naturally aligned.

/
    Leif