Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling controls to EL2

Andrew Murray
2019-12-23T12:10:46Z

On Mon, Dec 23, 2019 at 12:05:12PM +0000, Marc Zyngier wrote:
> On 2019-12-23 11:56, Andrew Murray wrote:
> > On Sun, Dec 22, 2019 at 10:42:05AM +0000, Marc Zyngier wrote:
> > > On Fri, 20 Dec 2019 14:30:18 +0000,
> > > Andrew Murray <andrew.murray@arm.com> wrote:
> > > >
> > > > As we now save/restore the profiler state there is no need to trap
> > > > accesses to the statistical profiling controls. Let's unset the
> > > > _TPMS bit.
> > > >
> > > > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > > ---
> > > > arch/arm64/kvm/debug.c | 2 --
> > > > 1 file changed, 2 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> > > > index 43487f035385..07ca783e7d9e 100644
> > > > --- a/arch/arm64/kvm/debug.c
> > > > +++ b/arch/arm64/kvm/debug.c
> > > > @@ -88,7 +88,6 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu
> > > *vcpu)
> > > > * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> > > > * - Debug ROM Address (MDCR_EL2_TDRA)
> > > > * - OS related registers (MDCR_EL2_TDOSA)
> > > > - * - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> > > > *
> > > > * Additionally, KVM only traps guest accesses to the debug
> > > registers if
> > > > * the guest is not actively using them (see the
> > > KVM_ARM64_DEBUG_DIRTY
> > > > @@ -111,7 +110,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu
> > > *vcpu)
> > > > */
> > > > vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) &
> > > MDCR_EL2_HPMN_MASK;
> > > > vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> > > > - MDCR_EL2_TPMS |
> > >
> > > No. This is an *optional* feature (the guest could not be presented
> > > with the SPE feature, or the the support simply not be compiled in).
> > >
> > > If the guest is not allowed to see the feature, for whichever
> > > reason,
> > > the traps *must* be enabled and handled.
> >
> > I'll update this (and similar) to trap such registers when we don't
> > support
> > SPE in the guest.
> >
> > My original concern in the cover letter was in how to prevent the guest
> > from attempting to use these registers in the first place - I think the
> > solution I was looking for is to trap-and-emulate ID_AA64DFR0_EL1 such
> > that
> > the PMSVer bits indicate that SPE is not emulated.
>
> That, and active trapping of the SPE system registers resulting in injection
> of an UNDEF into the offending guest.

Yes that's no problem.

Thanks,

Andrew Murray

>
> Thanks,
>
> M.
> --
> Jazz is not dead. It just smells funny...