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

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Message-ID: <1bb190091362262021dbaf41b5fe601e@www.loen.fr>
Date:   Mon, 23 Dec 2019 12:05:12 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Andrew Murray <andrew.murray@....com>
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 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.

Thanks,

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