Re: [PATCH v2 6/7] KVM: arm64: Upgrade PMU support to ARMv8.4

[Marc Zyngier <maz@kernel.org>]

On 2021-02-03 11:07, Auger Eric wrote:
> Hi Marc,
> On 2/3/21 11:36 AM, Marc Zyngier wrote:
>> Hi Eric,
>> 
>> On 2021-01-27 17:53, Auger Eric wrote:
>>> Hi Marc,
>>> 
>>> On 1/25/21 1:26 PM, Marc Zyngier wrote:
>>>> Upgrading the PMU code from ARMv8.1 to ARMv8.4 turns out to be
>>>> pretty easy. All that is required is support for PMMIR_EL1, which
>>>> is read-only, and for which returning 0 is a valid option as long
>>>> as we don't advertise STALL_SLOT as an implemented event.
>>>> 
>>>> Let's just do that and adjust what we return to the guest.
>>>> 
>>>> Signed-off-by: Marc Zyngier <maz@kernel.org>
>>>> ---
>>>>  arch/arm64/include/asm/sysreg.h |  3 +++
>>>>  arch/arm64/kvm/pmu-emul.c       |  6 ++++++
>>>>  arch/arm64/kvm/sys_regs.c       | 11 +++++++----
>>>>  3 files changed, 16 insertions(+), 4 deletions(-)
>>>> 
>>>> diff --git a/arch/arm64/include/asm/sysreg.h
>>>> b/arch/arm64/include/asm/sysreg.h
>>>> index 8b5e7e5c3cc8..2fb3f386588c 100644
>>>> --- a/arch/arm64/include/asm/sysreg.h
>>>> +++ b/arch/arm64/include/asm/sysreg.h
>>>> @@ -846,7 +846,10 @@
>>>> 
>>>>  #define ID_DFR0_PERFMON_SHIFT        24
>>>> 
>>>> +#define ID_DFR0_PERFMON_8_0        0x3
>>>>  #define ID_DFR0_PERFMON_8_1        0x4
>>>> +#define ID_DFR0_PERFMON_8_4        0x5
>>>> +#define ID_DFR0_PERFMON_8_5        0x6
>>>> 
>>>>  #define ID_ISAR4_SWP_FRAC_SHIFT        28
>>>>  #define ID_ISAR4_PSR_M_SHIFT        24
>>>> diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
>>>> index 398f6df1bbe4..72cd704a8368 100644
>>>> --- a/arch/arm64/kvm/pmu-emul.c
>>>> +++ b/arch/arm64/kvm/pmu-emul.c
>>>> @@ -795,6 +795,12 @@ u64 kvm_pmu_get_pmceid(struct kvm_vcpu *vcpu,
>>>> bool pmceid1)
>>>>          base = 0;
>>>>      } else {
>>>>          val = read_sysreg(pmceid1_el0);
>>>> +        /*
>>>> +         * Don't advertise STALL_SLOT, as PMMIR_EL0 is handled
>>>> +         * as RAZ
>>>> +         */
>>>> +        if (vcpu->kvm->arch.pmuver >= ID_AA64DFR0_PMUVER_8_4)
>>>> +            val &= ~BIT_ULL(ARMV8_PMUV3_PERFCTR_STALL_SLOT - 32);
>>> what about the STALL_SLOT_BACKEND and FRONTEND events then?
>> 
>> Aren't these a mandatory ARMv8.1 feature? I don't see a reason to
>> drop them.
> 
> I understand the 3 are linked together.
> 
> In D7.11 it is said
> "
> When any of the following common events are implemented, all three of
> them are implemented:
> 0x003D , STALL_SLOT_BACKEND, No operation sent for execution on a Slot
> due to the backend,
> 0x003E , STALL_SLOT_FRONTEND, No operation sent for execution on a Slot
> due to the frontend.
> 0x003F , STALL_SLOT, No operation sent for execution on a Slot.
> "

They are linked in the sense that they report related events, but they
don't have to be implemented in the same level of the architecure, if 
only
because BACKEND/FRONTEND were introducedway before ARMv8.4.

What the architecture says is:

- For FEAT_PMUv3p1 (ARMv8.1):
   "The STALL_FRONTEND and STALL_BACKEND events are required to be
    implemented." (A2.4.1, DDI0487G.a)

- For FEAT_PMUv3p4 (ARMv8.4):
   "If FEAT_PMUv3p4 is implemented:
    - If STALL_SLOT is not implemented, it is IMPLEMENTATION DEFINED 
whether the PMMIR System registers are implemented.
    - If STALL_SLOT is implemented, then the PMMIR System registers are 
implemented." (D7-2873, DDI0487G.a)

So while BACKEND/FRONTEND are required in an ARMv8.4 implementation
by virtue of being mandatory in ARMv8.1, STALL_SLOT isn't at any point.

Thanks,

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