On Thu, Sep 15, 2016 at 5:28 AM, Julien Grall <julien.grall@arm.com> wrote:

> Hello all,
>
> The ARM architecture mandates the use of a break-before-make sequence when
> changing translation entries if the page table is shared between multiple
> CPUs whenever a valid entry is replaced by another valid entry (see D4.7.1
> in ARM DDI 0487A.j for more details).
>
> The current P2M code does not respect this sequence and may result to
> break coherency on some processors.
>
> Adapting the current implementation to use break-before-make sequence would
> imply some code duplication and more TLBs invalidations than necessary.
> For instance, if we are replacing a 4KB page and the current mapping in
> the P2M is using a 1GB superpage, the following steps will happen:
>     1) Shatter the 1GB superpage into a series of 2MB superpages
>     2) Shatter the 2MB superpage into a series of 4KB superpages
>     3) Replace the 4KB page
>
> As the current implementation is shattering while descending and install
> the mapping before continuing to the next level, Xen would need to issue 3
> TLB invalidation instructions which is clearly inefficient.
>
> Furthermore, all the operations which modify the page table are using the
> same skeleton. It is more complicated to maintain different code paths than
> having a generic function that set an entry and take care of the
> break-before-
> make sequence.
>
> The new implementation is based on the x86 EPT one which, I think, fits
> quite well for the break-before-make sequence whilst keeping the code
> simple.
>
> For all the changes see in each patch.
>
> I have provided a branch based on upstream here:
> git://xenbits.xen.org/people/julieng/xen-unstable.git branch p2m-v2
>
>
Tested-by: Tamas K Lengyel <tamas@tklengyel.com>

Works without any issue on both the Cubietruck and the HiKey LeMaker with
the xen-access test-cases.

Cheers,
Tamas