Re: [PATCH RFC v2 0/2] arm: enable MTE for QEMU + kvm

[Dr. David Alan Gilbert]

* Cornelia Huck (cohuck@redhat.com) wrote:
> This series makes it possible to enable MTE for kvm guests, if the kernel
> supports it. Again, tested on the simulator via patiently waiting for the
> arm64/mte kselftests to finish successfully.
> 
> For tcg, turning on mte on the machine level (to get tag memory) stays a
> requirement. If the new mte cpu feature is not explicitly specified, a tcg
> vm will get mte depending on the presence of tag memory (just as today).
> 
> For kvm, mte stays off by default; this is because migration is not yet
> supported (postcopy will need an extension of the kernel interface, possibly
> an extension of the userfaultfd interface), and turning on mte will add a
> migration blocker.

My assumption was that a normal migration would need something as well
to retrieve and place the MTE flags; albeit not atomically.

> My biggest question going forward is actually concerning migration; I gather
> that we should not bother adding something unless postcopy is working as well?

I don't think that restriction is fair on you; just make sure
postcopy_ram_supported_by_host gains an arch call and fails cleanly;
that way if anyone tries to enable postcopy they'll find out with a
clean fail.

> If I'm not misunderstanding things, we need a way to fault in a page together
> with the tag; doing that in one go is probably the only way that we can be
> sure that this is race-free on the QEMU side. Comments welcome :)

I think it will.
But, ignoring postcopy for a minute, with KVM how do different types of
backing memory work - e.g. if I back a region of guest memory with
/dev/shm/something or a hugepage equivalent, where does the MTE memory
come from, and how do you set it?

Dave

> Changes v1->v2: [Thanks to Eric for the feedback!]
> - add documentation
> - switch the mte prop to OnOffAuto; this improves the interaction with the
>   existing mte machine prop
> - leave mte off for kvm by default
> - improve tests; the poking in QDicts feels a bit ugly, but seems to work
> 
> Cornelia Huck (2):
>   arm/kvm: add support for MTE
>   qtests/arm: add some mte tests
> 
>  docs/system/arm/cpu-features.rst |  21 +++++
>  target/arm/cpu.c                 |  18 ++---
>  target/arm/cpu.h                 |   1 +
>  target/arm/cpu64.c               | 132 +++++++++++++++++++++++++++++++
>  target/arm/internals.h           |   1 +
>  target/arm/kvm64.c               |   5 ++
>  target/arm/kvm_arm.h             |  12 +++
>  target/arm/monitor.c             |   1 +
>  tests/qtest/arm-cpu-features.c   |  77 ++++++++++++++++++
>  9 files changed, 256 insertions(+), 12 deletions(-)
> 
> -- 
> 2.35.3
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK