On 29 June 2016 at 19:00, Leif Lindholm [off-list ref] wrote:

On Wed, Jun 29, 2016 at 06:53:18PM +0200, Ard Biesheuvel wrote:

quoted

On 29 June 2016 at 18:50, Ard Biesheuvel [off-list ref] wrote:

quoted

On 29 June 2016 at 18:45, Catalin Marinas [off-list ref] wrote:

quoted

On Wed, Jun 29, 2016 at 02:51:28PM +0200, Ard Biesheuvel wrote:

quoted

+     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?