Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear_debug()

Christoffer Dall
2015-04-13T14:40:53Z

On Mon, Apr 13, 2015 at 04:36:23PM +0200, Christoffer Dall wrote:

[...]

> > +
> > +/**
> > + * kvm_arch_setup_debug - set-up debug related stuff
>
> nit: I think you want "set up" when it's a verb.
>
> > + *
> > + * @vcpu: the vcpu pointer
> > + *
> > + * This is called before each entry in to the hypervisor to setup any
>
> s/in to/into/
> s/setup/set up/
>
> > + * debug related registers. Currently this just ensures we will trap
> > + * access to:
>
> guest accesses to:
>
> > + * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> > + * - Debug ROM Address (MDCR_EL2_TDRA)
> > + * - Power down debug registers (MDCR_EL2_TDOSA)
> > + *
> > + * Additionally the hypervisor lazily saves/restores the debug
> > + * register state. If it is not currently doing so (arch.debug_flags)
> > + * then we also need to ensure we trap if the guest messes with them
> > + * so we know we need to save them.
>
> This paragraph is a little hard to make sense of. If I understand it
> correctly, the point is that when debugging the guest we need to make
> sure guest accesses to the debug registers traps? If so, I would
> suggest something like:
>
> Additionally, KVM only traps guest accesses to the debug registers if
> the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> flag on vcpu->arch.debug_flags). Since the guest must not interfere
> with the hardware state when debugging the guest, we must ensure that
> trapping is enabled whenever we are debugging the guest.
>

thinking about this, I don't think we're enforcing this yet, but maybe
that will come in the later patches or I misread the original paragraph.

-Christoffer