Re: [RFC PATCH v3 09/29] KVM: arm64: Hide IMPLEMENTATION DEFINED PMU support for the guest

From: Eric Auger <eauger@redhat.com>
To: Reiji Watanabe <reijiw@google.com>
Cc: kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Peter Shier <pshier@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH v3 09/29] KVM: arm64: Hide IMPLEMENTATION DEFINED PMU support for the guest
Date: Thu, 2 Dec 2021 11:57:11 +0100	[thread overview]
Message-ID: <5bd01c9c-6ac8-4034-6f49-be636a3b287c@redhat.com> (raw)
In-Reply-To: <CAAeT=FzM=sLF=PkY_shhcYmfo+ReGEBN8XX=QQObavXDtwxFJQ@mail.gmail.com>

Hi Reiji,

On 11/30/21 6:32 AM, Reiji Watanabe wrote:
> Hi Eric,
> 
> On Thu, Nov 25, 2021 at 12:30 PM Eric Auger <eauger@redhat.com> wrote:
>>
>> Hi Reiji,
>>
>> On 11/17/21 7:43 AM, Reiji Watanabe wrote:
>>> When ID_AA64DFR0_EL1.PMUVER or ID_DFR0_EL1.PERFMON is 0xf, which
>>> means IMPLEMENTATION DEFINED PMU supported, KVM unconditionally
>>> expose the value for the guest as it is.  Since KVM doesn't support
>>> IMPLEMENTATION DEFINED PMU for the guest, in that case KVM should
>>> exopse 0x0 (PMU is not implemented) instead.
>> s/exopse/expose
>>>
>>> Change cpuid_feature_cap_perfmon_field() to update the field value
>>> to 0x0 when it is 0xf.
>> is it wrong to expose the guest with a Perfmon value of 0xF? Then the
>> guest should not use it as a PMUv3?
> 
>> is it wrong to expose the guest with a Perfmon value of 0xF? Then the
>> guest should not use it as a PMUv3?
> 
> For the value 0xf in ID_AA64DFR0_EL1.PMUVER and ID_DFR0_EL1.PERFMON,
> Arm ARM says:
>   "IMPLEMENTATION DEFINED form of performance monitors supported,
>    PMUv3 not supported."
> 
> Since the PMU that KVM supports for guests is PMUv3, 0xf shouldn't
> be exposed to guests (And this patch series doesn't allow userspace
> to set the fields to 0xf for guests).
What I don't get is why this isn't detected before (in kvm_reset_vcpu).
if the VCPU was initialized with KVM_ARM_VCPU_PMU_V3 can we honor this
init request if the host pmu is implementation defined?

Thanks

Eric
> 
> Thanks,
> Reiji
> 
>>
>> Eric
>>>
>>> Fixes: 8e35aa642ee4 ("arm64: cpufeature: Extract capped perfmon fields")
>>> Signed-off-by: Reiji Watanabe <reijiw@google.com>
>>> ---
>>>  arch/arm64/include/asm/cpufeature.h | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>>> index ef6be92b1921..fd7ad8193827 100644
>>> --- a/arch/arm64/include/asm/cpufeature.h
>>> +++ b/arch/arm64/include/asm/cpufeature.h
>>> @@ -553,7 +553,7 @@ cpuid_feature_cap_perfmon_field(u64 features, int field, u64 cap)
>>>
>>>       /* Treat IMPLEMENTATION DEFINED functionality as unimplemented */
>>>       if (val == ID_AA64DFR0_PMUVER_IMP_DEF)
>>> -             val = 0;
>>> +             return (features & ~mask);
>>>
>>>       if (val > cap) {
>>>               features &= ~mask;
>>>
>>
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