[RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running

On 8 July 2015 at 17:37, Marc Zyngier [off-list ref] wrote:

On 08/07/15 17:06, Peter Maydell wrote:

quoted

I'd prefer it if somebody could investigate to see why QEMU
is actually doing this -- so far we just have speculation.

I'd prefer that too, but so far people seem to be more comfortable
waiting for the issue to fix itself. In the meantime, VMs are broken in
weird and wonderful ways, and I don't think the current status-quo helps
anyone.

Putting in a patch which might not be the right fix isn't
necessarily a good plan either...

Does has_run_once get cleared if we do a re-VCPU_INIT
of a CPU that's run before? (We need to allow rewriting
of guest state at that point so that "reset VM and
load migration state" behaves correctly.)

I suspect Jan is right and we really need to distinguish
the KVM_PUT_*_STATE levels in ARM QEMU. This probably
implies some kind of whitelist/override mechanism, since
by and large we neither know nor want to know the
semantics for system registers, we leave that up to the
kernel.

Q: if you have a running VM, and you pause it for
an hour, what should the CNTVCT register do? Presumably
it should not advance, but how do we arrange for that
to happen?

-- PMM