[Xen-devel] [PATCH 0/4] xen/arm: fix guest builder cache cohenrency (again, again)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

Jan/Ian/Keir -- the final patch involves tools changes and a new domctl,
which is why you are copied (although the domctl is marked as arm
specific you might have opinions on it).

On ARM we need to take care of cache coherency for guests which we have
just built because they start with their caches disabled.

Our current strategy for dealing with this, which is to make guest
memory default to cacheable regardless of the in guest configuration
(the HCR.DC bit), is flawed because it doesn't handle guests which
enable their MMU before enabling their caches, which at least FreeBSD
does. (NB: Setting HCR.DC while the guest MMU is enabled is
UNPREDICTABLE, hence we must disable it when the guest turns its MMU
one).

There is also a security aspect here since the current strategy means
that a guest which enables its MMU before its caches can potentially see
unscrubbed data in RAM (because the scrubbed bytes are still held in the
cache).

As well as the new stuff this series removes the HCR.DC support and
performs two purely cosmetic renames.

This has survived 20000 bootloops on arm32 and 9700 (on going) on arm64.
(I've been seeing a sporadic issue on arm64 but I believe it is
unrelated, although I also cannot reproduce at the moment).

As well as being more correct (and secure!) this strategy is IMHO
simpler than the existing HCR.DC based thing and I'd like to push it to
4.4.

I'm aware that this is the third attempt to get this right and the
second one requiring a freeze exception :-(

Ian.


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