linux-kernel - Re: [RFC 2/3] KVM: arm64: pmu: Fix chained SW

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Message-ID: <15507faca89a980056df7119e105e82a@www.loen.fr>
Date:   Thu, 05 Dec 2019 14:52:26 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     <eric.auger.pro@...il.com>, <linux-kernel@...r.kernel.org>,
        <kvmarm@...ts.cs.columbia.edu>, <james.morse@....com>,
        <andrew.murray@....com>, <suzuki.poulose@....com>,
        <drjones@...hat.com>
Subject: Re: [RFC 2/3] KVM: arm64: pmu: Fix chained SW_INCR counters

On 2019-12-05 14:06, Auger Eric wrote:
> Hi Marc,
>
> On 12/5/19 10:43 AM, Marc Zyngier wrote:
>> Hi Eric,
>>
>> On 2019-12-04 20:44, Eric Auger wrote:
>>> At the moment a SW_INCR counter always overflows on 32-bit
>>> boundary, independently on whether the n+1th counter is
>>> programmed as CHAIN.
>>>
>>> Check whether the SW_INCR counter is a 64b counter and if so,
>>> implement the 64b logic.
>>>
>>> Fixes: 80f393a23be6 ("KVM: arm/arm64: Support chained PMU 
>>> counters")
>>> Signed-off-by: Eric Auger <eric.auger@...hat.com>
>>> ---
>>>  virt/kvm/arm/pmu.c | 16 +++++++++++++++-
>>>  1 file changed, 15 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
>>> index c3f8b059881e..7ab477db2f75 100644
>>> --- a/virt/kvm/arm/pmu.c
>>> +++ b/virt/kvm/arm/pmu.c
>>> @@ -491,6 +491,8 @@ void kvm_pmu_software_increment(struct kvm_vcpu
>>> *vcpu, u64 val)
>>>
>>>      enable = __vcpu_sys_reg(vcpu, PMCNTENSET_EL0);
>>>      for (i = 0; i < ARMV8_PMU_CYCLE_IDX; i++) {
>>> +        bool chained = test_bit(i >> 1, vcpu->arch.pmu.chained);
>>> +
>>
>> I'd rather you use kvm_pmu_pmc_is_chained() rather than open-coding
>> this. But see below:
>>
>>>          if (!(val & BIT(i)))
>>>              continue;
>>>          type = __vcpu_sys_reg(vcpu, PMEVTYPER0_EL0 + i)
>>> @@ -500,8 +502,20 @@ void kvm_pmu_software_increment(struct 
>>> kvm_vcpu
>>> *vcpu, u64 val)
>>>              reg = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) + 1;
>>>              reg = lower_32_bits(reg);
>>>              __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i) = reg;
>>> -            if (!reg)
>>> +            if (reg) /* no overflow */
>>> +                continue;
>>> +            if (chained) {
>>> +                reg = __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i + 1) 
>>> + 1;
>>> +                reg = lower_32_bits(reg);
>>> +                __vcpu_sys_reg(vcpu, PMEVCNTR0_EL0 + i + 1) = reg;
>>> +                if (reg)
>>> +                    continue;
>>> +                /* mark an overflow on high counter */
>>> +                __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(i + 1);
>>> +            } else {
>>> +                /* mark an overflow */
>>>                  __vcpu_sys_reg(vcpu, PMOVSSET_EL0) |= BIT(i);
>>> +            }
>>>          }
>>>      }
>>>  }
>>
>> I think the whole function is a bit of a mess, and could be better
>> structured to treat 64bit counters as a first class citizen.
>>
>> I'm suggesting something along those lines, which tries to
>> streamline things a bit and keep the flow uniform between the
>> two word sizes. IMHO, it helps reasonning about it and gives
>> scope to the ARMv8.5 full 64bit counters... It is of course
>> completely untested.
>
> Looks OK to me as well. One remark though, don't we need to test if 
> the
> n+1th reg is enabled before incrementing it?

Hmmm. I'm not sure. I think we should make sure that we don't flag
a counter as being chained if the odd counter is disabled, rather
than checking it here. As long as the odd counter is not chained
*and* enabled, we shouldn't touch it.

Again, untested:

diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
index cf371f643ade..47366817cd2a 100644
--- a/virt/kvm/arm/pmu.c
+++ b/virt/kvm/arm/pmu.c
@@ -15,6 +15,7 @@
  #include <kvm/arm_vgic.h>

  static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 
select_idx);
+static void kvm_pmu_update_pmc_chained(struct kvm_vcpu *vcpu, u64 
select_idx);

  #define PERF_ATTR_CFG1_KVM_PMU_CHAINED 0x1

@@ -298,6 +299,7 @@ void kvm_pmu_enable_counter_mask(struct kvm_vcpu 
*vcpu, u64 val)
  		 * For high counters of chained events we must recreate the
  		 * perf event with the long (64bit) attribute set.
  		 */
+		kvm_pmu_update_pmc_chained(vcpu, i);
  		if (kvm_pmu_pmc_is_chained(pmc) &&
  		    kvm_pmu_idx_is_high_counter(i)) {
  			kvm_pmu_create_perf_event(vcpu, i);
@@ -645,7 +647,8 @@ static void kvm_pmu_update_pmc_chained(struct 
kvm_vcpu *vcpu, u64 select_idx)
  	struct kvm_pmu *pmu = &vcpu->arch.pmu;
  	struct kvm_pmc *pmc = &pmu->pmc[select_idx];

-	if (kvm_pmu_idx_has_chain_evtype(vcpu, pmc->idx)) {
+	if (kvm_pmu_idx_has_chain_evtype(vcpu, pmc->idx) &&
+	    kvm_pmu_counter_is_enabled(vcpu, pmc->idx)) {
  		/*
  		 * During promotion from !chained to chained we must ensure
  		 * the adjacent counter is stopped and its event destroyed

What do you think?

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