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

[Marc Zyngier <maz@kernel.org>]

On 2021-02-04 12:32, Alexandru Elisei wrote:
> Hi,
> 
> On 2/3/21 1:28 PM, Marc Zyngier wrote:
>> On 2021-02-03 12:39, Auger Eric wrote:
>>> Hi,
>>> 
>>> On 2/3/21 12:20 PM, Marc Zyngier wrote:
>>>> 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)
>>> OK
>>>> 
>>>> - 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.
>>> But then how do you understand "When any of the following common 
>>> events
>>> are implemented, all three of them are implemented"?
>> 
>> I think that's wholly inconsistent, because it would mean that 
>> STALL_SLOT
>> isn't optional on ARMv8.4, and would make PMMIR mandatory.
> 
> I think there's some confusion regarding the event names. From my 
> reading of the
> architecture, STALL != STALL_SLOT, STALL_BACKEND != STALL_SLOT_BACKEND,
> STALL_FRONTEND != STALL_SLOT_FRONTEND.

Dammit, I hadn't realised that at all. Thanks for putting me right.

> 
> STALL{, _BACKEND, _FRONTEND} count the number of CPU cycles where no
> instructions
> are being executed on the PE (page D7-2872), STALL_SLOT{, _BACKEND, 
> _FRONTEND}
> count the number of slots where no instructions are being executed
> (page D7-2873).
> 
> STALL_{BACKEND, FRONTEND} are required by ARMv8.1-PMU (pages A2-76, 
> D7-2913);
> STALL is required by ARMv8.4-PMU (page D7-2914).
> 
> STALL_SLOT{, _BACKEND, _FRONTEND} are optional in ARMv8.4-PMU (pages 
> D7-2913,
> D7-2914), but if one of them is implemented, all of them must be
> implemented (page D7-2914).
> 
> The problem I see with this patch is that it doesn't clear the
> STALL_SLOT_{BACKEND, FRONTEND} event bits along with the STALL_SLOT bit 
> from
> PMCEID1_EL0.

OK, so Eric was right, and I'm an illiterate idiot! I'll fix the patch 
and repost
the whole thing.

Thanks,

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