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

On Thu, Jul 09, 2015 at 01:07:24PM +0100, Peter Maydell wrote:

On 9 July 2015 at 13:05, Christoffer Dall [off-list ref] wrote:

quoted

As I understand it, the problem is that if we ever run a VCPU after
reading the value, and write back the value afterwards, you potentially
make time go backwards and get inconsistent views of time from different
VCPUs because they may have read the time before/after updating the
CNTVOFF.

Right, but I think if QEMU does that it's a bug (and more to
the point I don't entirely understand why we would do that
yet, even given that we don't have a distinction between
"registers to sync always" and "registers to sync only on
reset"...)

I think we have evidence that it does that, but we don't know why/how.

-Christoffer