Re: [PATCH v4 12/16] KVM: arm64: PMU: Allow ID_DFR0_EL1.PerfMon to be set from userspace

From: Marc Zyngier <maz@kernel.org>
To: Reiji Watanabe <reijiw@google.com>
Cc: linux-arm-kernel@lists.infradead.org,
	kvmarm@lists.cs.columbia.edu, kvmarm@lists.linux.dev,
	kvm@vger.kernel.org, James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Ricardo Koller <ricarkol@google.com>
Subject: Re: [PATCH v4 12/16] KVM: arm64: PMU: Allow ID_DFR0_EL1.PerfMon to be set from userspace
Date: Sat, 19 Nov 2022 12:54:02 +0000	[thread overview]
Message-ID: <6b70f8fac8b2fd316b618c3c2e67e07b@kernel.org> (raw)
In-Reply-To: <CAAeT=Fwq8wcBvoWE+5RJt4og4uD5KgQ0kzhDZorGPjHmTguwzw@mail.gmail.com>

On 2022-11-19 05:52, Reiji Watanabe wrote:
> Hi Marc,
> 
> On Sun, Nov 13, 2022 at 8:46 AM Marc Zyngier <maz@kernel.org> wrote:
>> 
>> Allow userspace to write ID_DFR0_EL1, on the condition that only
>> the PerfMon field can be altered and be something that is compatible
>> with what was computed for the AArch64 view of the guest.
>> 
>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>> ---
>>  arch/arm64/kvm/sys_regs.c | 57 
>> ++++++++++++++++++++++++++++++++++++++-
>>  1 file changed, 56 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 3cbcda665d23..dc201a0557c0 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -1070,6 +1070,19 @@ static u8 vcpu_pmuver(const struct kvm_vcpu 
>> *vcpu)
>>         return vcpu->kvm->arch.dfr0_pmuver.unimp;
>>  }
>> 
>> +static u8 perfmon_to_pmuver(u8 perfmon)
>> +{
>> +       switch (perfmon) {
>> +       case ID_DFR0_PERFMON_8_0:
>> +               return ID_AA64DFR0_EL1_PMUVer_IMP;
>> +       case ID_DFR0_PERFMON_IMP_DEF:
>> +               return ID_AA64DFR0_EL1_PMUVer_IMP_DEF;
> 
> Nit: Since IMP_DEF is 0xf for both PMUVER and PERFMON,
> I think the 'default' can handle IMP_DEF (I have the same
> comment for pmuver_to_perfmon in the patch-10).

It sure can, but IMP_DEF is special enough in its treatment
(we explicitly check this value in set_id_dfr0_el1()) that
it actually helps the reader to keep the explicit conversion
here.

> 
>> +       default:
>> +               /* Anything ARMv8.1+ has the same value. For now. */
> 
> Nit: Shouldn't the comment also mention NI (and IMP_DEF) ?
> (I have the same comment for pmuver_to_perfmon in the patch-10)

I can expand the comment to include NI.

> Otherwise:
> Reviewed-by: Reiji Watanabe <reijiw@google.com>

Thanks,

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

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