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Re: [Xen-devel] [for-4.8][PATCH v2 00/23] xen/arm: Rework the P2M code to follow break-before-make sequence

From: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>

Date: Thu, 15 Sep 2016 11:23:03 -0600

Cc: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, Steve Capper <steve.capper@xxxxxxx>, Sergej Proskurin <proskurin@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxx>, Dirk Behme <dirk.behme@xxxxxxxxxxxx>, Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>, wei.chen@xxxxxxxxxx

Delivery-date: Thu, 15 Sep 2016 17:23:24 +0000

List-id: Xen developer discussion <xen-devel.lists.xen.org>

On Thu, Sep 15, 2016 at 5:28 AM, Julien Grall <julien.grall@xxxxxxx> 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@xxxxxxxxxxxxx>

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

Cheers,

Tamas

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