Re: [PATCH v2 0/9] arm64: KVM: debug infrastructure support

Christoffer Dall
2014-05-28T09:58:37Z

On Wed, May 28, 2014 at 10:56:45AM +0100, Marc Zyngier wrote:
> On 25/05/14 16:34, Christoffer Dall wrote:
> > On Tue, May 20, 2014 at 05:55:36PM +0100, Marc Zyngier wrote:
> >> This patch series adds debug support, a key feature missing from the
> >> KVM/arm64 port.
> >>
> >> The main idea is to keep track of whether the debug registers are
> >> "dirty" (changed by the guest) or not. In this case, perform the usual
> >> save/restore dance, for one run only. It means we only have a penalty
> >> if a guest is actively using the debug registers.
> >>
> >> The amount of registers is properly frightening, but CPUs actually
> >> only implement a subset of them. Also, there is a number of registers
> >> we don't bother emulating (things having to do with external debug and
> >> OSlock).
> >
> > What is the rationale about not having to deal with external debug and
> > OSlock?
>
> External debug is when you actually plug a physical JTAG into the CPU.
> OSlock is a way to prevent "other software" to play with the debug
> registers. My understanding is that it is only useful in combination
> with the external debug.
>
> In both case, implementing support for this is probably not worth the
> effort, at least for the time being.
>
OK, can we document that somewhere clearly in the code then so we know
how we can simply ignore those registers?

Thanks,
-Christoffer