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

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Date:   Mon, 23 Dec 2019 12:10:42 +0000
From:   Andrew Murray <andrew.murray@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 11/18] KVM: arm64: don't trap Statistical Profiling
 controls to EL2

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@....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@....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...