[PATCH v2 07/12] ARM: split off core mapping logic from create_mapping

On Mon, Nov 16, 2015 at 08:01:03PM +0100, Ard Biesheuvel wrote:

On 16 November 2015 at 19:55, Russell King - ARM Linux
[off-list ref] wrote:

quoted

I'm slightly worried about this.  Generally, these functions setup
global mappings.  If you're wanting to have a private set of page
tables for UEFI, and those private page tables contain global
mappings which are different from the mappings in the kernel's page
tables, then you need careful break-TLBflush-make handling when
switching from the kernel's page tables to the private UEFI ones,
and vice versa.

Has this aspect been considered?

Yes. The UEFI mappings are all below 1 GB, and the top kernel end is
reused as we do for ordinary userland page tables. The switch to the
UEFI page tables performs a full TLB flush. What it does not do is
break before make, as far as I can tell, so any pointers regarding how
to implement that would be appreciated (this is implemented in 11/12)

What matters is whether they are global mappings or not.  If they are,
when the TLB can contain conflicting entries.  If they are non-global
mappings, they will be tagged with the ASID which makes them unique
to each mm.

The simple solution is to ensure that they're non-global, and then you
don't need to even worry about flushing the TLB when switching.

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