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

[Peter Maydell]

On 8 July 2015 at 16:56, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 29/06/15 18:37, Peter Maydell wrote:
>> On 29 June 2015 at 18:20, Claudio Fontana <claudio.fontana at huawei.com> wrote:
>>> On 26.06.2015 06:49, Jan Kiszka wrote:
>>>> QEMU has the concept of write-back levels: KVM_PUT_RUNTIME_STATE,
>>>> KVM_PUT_RESET_STATE and KVM_PUT_FULL_STATE. I suspect this registers is
>>>> just sorted into the wrong category, thus written as part of the
>>>> RUNTIME_STATE. We had such bug patterns during the x86 maturing phase as
>>>> well.
>>
>>> It seems that QEMU target-arm ignores the level parameter to
>>> kvm_arch_put_registers completely.
>>>
>>> Is it intended?
>>
>> Yes, sort of. We don't in general know anything about the semantics
>> of most of the system registers. It should always be safe to
>> read them all out of the kernel and write them back...
>
> I'm not sure you can safely assume this for time related things, unless
> you can guarantee that all vcpus are stopped. Claudio is seeing time
> jumping in weird ways, and so have I, which would tend to show that QEMU
> is introducing some jitter.
>
> Maybe not easily observable on real hardware, but the FastModel is
> enough to show the issue.
>
> So unless someone has a better solution, I'm seriously considering
> getting this patch merged.

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

-- PMM