Re: [PATCH v6 10/21] KVM: ARM64: Add access handler for PMEVCNTRn and PMCCNTR register

[Shannon Zhao <zhaoshenglong@huawei.com>]

On 2015/12/10 20:07, Marc Zyngier wrote:
> Hi Shannon,
> 
> On 10/12/15 11:36, Shannon Zhao wrote:
>> > Hi Marc,
>> > 
>> > On 2015/12/9 0:30, Marc Zyngier wrote:
>>> >> On 08/12/15 12:47, Shannon Zhao wrote:
>>>>> >>>> From: Shannon Zhao <shannon.zhao@linaro.org>
>>>>> >>>>
>>>>> >>>> Since the reset value of PMEVCNTRn or PMCCNTR is UNKNOWN, use
>>>>> >>>> reset_unknown for its reset handler. Add access handler which emulates
>>>>> >>>> writing and reading PMEVCNTRn or PMCCNTR register. When reading
>>>>> >>>> PMEVCNTRn or PMCCNTR, call perf_event_read_value to get the count value
>>>>> >>>> of the perf event.
>>>>> >>>>
>>>>> >>>> Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
>>>>> >>>> ---
>>>>> >>>>  arch/arm64/kvm/sys_regs.c | 107 +++++++++++++++++++++++++++++++++++++++++++++-
>>>>> >>>>  1 file changed, 105 insertions(+), 2 deletions(-)
>>>>> >>>>
>>>>> >>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>>>> >>>> index c116a1b..f7a73b5 100644
>>>>> >>>> --- a/arch/arm64/kvm/sys_regs.c
>>>>> >>>> +++ b/arch/arm64/kvm/sys_regs.c
>>>>> >>>> @@ -525,6 +525,12 @@ static bool access_pmu_regs(struct kvm_vcpu *vcpu,
>>>>> >>>>  
>>>>> >>>>  	if (p->is_write) {
>>>>> >>>>  		switch (r->reg) {
>>>>> >>>> +		case PMEVCNTR0_EL0 ... PMCCNTR_EL0: {
>>> >> Same problem as previously mentioned.
>>> >>
>>>>> >>>> +			val = kvm_pmu_get_counter_value(vcpu,
>>>>> >>>> +							r->reg - PMEVCNTR0_EL0);
>>>>> >>>> +			vcpu_sys_reg(vcpu, r->reg) += (s64)p->regval - val;
>>>>> >>>> +			break;
>>>>> >>>> +		}
>> > 
>> > If I use a handler to handle these accesses to PMEVCNTRn and PMCCNTR
>> > like below. It converts the register offset c14_PMEVCNTRn and c9_PMCCNTR
>> > to PMEVCNTRn_EL0 and PMCCNTR_EL0, uniformly uses vcpu_sys_reg and
>> > doesn't need to take care the big endian. What do you think about this?
>> > 
>> > static bool access_pmu_evcntr(struct kvm_vcpu *vcpu,
>> >                               struct sys_reg_params *p,
>> >                               const struct sys_reg_desc *r)
>> > {
>> >         u64 idx, reg, val;
>> > 
>> >         if (p->is_aarch32)
>> >                 reg = r->reg / 2;
> I'd prefer it if you actually decoded the reg itself. Something like:
> 
> 	if (p->is_aarch32) {
> 		if (r->CRn == 9 && r->CRm == 13)
> 			reg = (r->Op2 & 1) ? 0 : PMCCNTR_EL0;
> 		if (r->CRn == 14 && (r->CRm & 0xc) == 8) {
> 			reg = ((r->CRm & 3) << 2) & (r->Op2 & 7);
> 			reg += PMEVCNTR0_EL0;
> 		} else {
> 			BUG();
> 		}
> 	} else {
> 		....
> 	}
> 
> And then you can get rid of the c14_PMVCNTR* and c9_PMCCNTR macros.
> The only slightly ugly thing is this 0 value to represent PMXEVTYPER,
> but that's what we already have with your "default" clause below.
> 
Ok, thanks.
>> >         else
>> >                 reg = r->reg;
>> > 
>> >         switch (reg) {
>> >         case PMEVCNTR0_EL0 ... PMEVCNTR30_EL0: {
>> >                 idx = reg - PMEVCNTR0_EL0;
>> >                 break;
>> >         }
>> >         case PMCCNTR_EL0: {
>> >                 idx = ARMV8_CYCLE_IDX;
>> >                 break;
>> >         }
>> >         default:
>> >                 idx = vcpu_sys_reg(vcpu, PMSELR_EL0) & ARMV8_COUNTER_MASK;
>> >                 if (!pmu_counter_idx_valid(vcpu, idx))
>> >                         return true;
>> >                 reg = (idx == ARMV8_CYCLE_IDX) ? PMCCNTR_EL0 :
>> > PMEVCNTR0_EL0 + idx;
>> >                 break;
>> >         }
>> > 
>> >         val = kvm_pmu_get_counter_value(vcpu, idx);
>> >         if (p->is_write)
>> >                 vcpu_sys_reg(vcpu, reg) = (s64)p->regval - val;
> Maybe I don't have my head screwed in the right way, but as long as
> we're only using u64 quantities, why do we need this s64 cast?
> 
In case p->regval less than val.
For example, at the first time it writes 0x80000001 to
vcpu_sys_reg(vcpu, reg) and start a perf event which counts a value
10000. Then we want to start the same perf event with writing 0x80000001
to vcpu_sys_reg(vcpu, reg) too. At this time, p->regval(0x80000001) is
less than val(0x80000001 + 10000).

Thanks,
-- 
Shannon