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

[Alexander Graf]

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 at 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.


Alex