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

[Marc Zyngier]

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@xxxxxxx> 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@xxxxxxx>
> > > ---
> > >  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.

Thanks,

        M.
--
Jazz is not dead. It just smells funny...