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

Christoffer Dall christoffer.dall at linaro.org
Thu Jun 20 18:48:44 EDT 2013

On Fri, Jun 21, 2013 at 12:02:02AM +0200, Alexander Graf wrote:
> On 20.06.2013, at 23:59, Peter Maydell wrote:
> > On 20 June 2013 22:55, Alexander Graf <agraf at 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 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.
> > 
> > Doesn't protect you against confusion due to another thread running
> > a different vcpu in the same vm, though.
> Ah, different ONE_REG API. Can't you just notify all vcpus to exit and refresh their timers? That's what kvm_make_request() is there for, no?
yes you can, but I don't think it's worth the trouble to add the code in
the kernel to fix a case where user space does something completely
broken, which does not muck with the hardware or host state, but can
only break the guest.

I didn't realize that ONE_REG does vcpu_load() (or, I probably did once,
and forgot) so that means we're good.

Conclusion on this patch: address Marc's comment to move the user space
interface handling out of arch_timer.c and we should be good.


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