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

[Cornelia Huck <cohuck@redhat.com>]

On Mon, Jul 11 2022, "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Cornelia Huck (cohuck@redhat.com) wrote:
>> 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.

There's KVM_ARM_MTE_COPY_TAGS, which should be sufficient to move tags
around for normal migration.

>
>> 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.

Ok, if simply fencing off postcopy is fine, we can try to move forward
with what we have now. The original attempt at
https://lore.kernel.org/all/881871e8394fa18a656dfb105d42e6099335c721.1615972140.git.haibo.xu@linaro.org/
hooked itself directly into common code; maybe we should rather copy the
approach used for s390 storage keys (extra "device") instead?