[PATCH v2] ARM/KVM: save and restore generic timer registers

[peter.maydell@linaro.org (Peter Maydell)]

On 20 June 2013 22:55, Alexander Graf <agraf@suse.de> wrote:
>
> On 20.06.2013, at 22:37, Christoffer Dall wrote:
>
>> On Thu, Jun 20, 2013 at 08:29:30PM +0100, Peter Maydell wrote:
>>> On 20 June 2013 19:32, Christoffer Dall <christoffer.dall@linaro.org> wrote:
>>>> Marc wrote:
>>>>> So there is just one thing we absolutely need to make sure here: no vcpu
>>>>> can run before they've all had their timer restored, and hence a stable
>>>>> cntvoff. Otherwise two vcpus will have a different view of time.
>>>>>
>>>>> Can we guarantee this?
>>>
>>>> Do we need to?  User space is free to modify time and all sort of other
>>>> registers at any point during VM execution - it will just break the
>>>> guest that it's running.
>>>
>>> Note that QEMU will stop all CPUs before doing a migration or
>>> similar operation. However there is a monitor command to query
>>> the current CPU registers etc which won't try to stop the VM
>>> first. So we might try to read vcpu registers (though I hope we
>>> don't allow writing them).
>>>
>> Sounds like we need to add a -EBUSY return on SET_ONE_REG if the VM is
>> running.
>
> The ONE_REG API should already be protected here, as it does
> vcpu_load() in kvm_vcpu_ioctl(). So a separate thread can't possibly
> do ONE_REG accesses while another thread has the same vcpu running.

Doesn't protect you against confusion due to another thread running
a different vcpu in the same vm, though.

thanks
-- PMM