Subject: Re: [PATCH v2 1/6] arm64: add support for the AMU extension v1

From: Ionela Voinescu <[email-masked]>
Date: Thu, 23 Jan 2020 18:32:07 +0000

On Thursday 23 Jan 2020 at 17:04:07 (+0000), Valentin Schneider wrote:
> Hi Ionela,
> 
> On 18/12/2019 18:26, Ionela Voinescu wrote:
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -382,6 +382,42 @@
> >  #define SYS_TPIDR_EL0			sys_reg(3, 3, 13, 0, 2)
> >  #define SYS_TPIDRRO_EL0			sys_reg(3, 3, 13, 0, 3)
> >  
> > +/* Definitions for system register interface to AMU for ARMv8.4 onwards */
> > +#define SYS_AM_EL0(crm, op2)		sys_reg(3, 3, 13, crm, op2)
> > +#define SYS_AMCR_EL0			SYS_AM_EL0(2, 0)
> > +#define SYS_AMCFGR_EL0			SYS_AM_EL0(2, 1)
> > +#define SYS_AMCGCR_EL0			SYS_AM_EL0(2, 2)
> > +#define SYS_AMUSERENR_EL0		SYS_AM_EL0(2, 3)
> > +#define SYS_AMCNTENCLR0_EL0		SYS_AM_EL0(2, 4)
> > +#define SYS_AMCNTENSET0_EL0		SYS_AM_EL0(2, 5)
> > +#define SYS_AMCNTENCLR1_EL0		SYS_AM_EL0(3, 0)
> > +#define SYS_AMCNTENSET1_EL0		SYS_AM_EL0(3, 1)
> > +
> > +/*
> > + * Group 0 of activity monitors (architected):
> > + *                op0 CRn   op1   op2     CRm
> > + * Counter:       11  1101  011   n<2:0>  010:n<3>
> 
> Nit: any reason for picking a different order than the encoding one? e.g.
>                      op0  op1  CRn   CRm       op2
>                      11   011  1101  010:<n3>  n<2:0>

I followed the format in the documentation at the time: DDI 0487D.a.
But you are correct as in I should have used the encoding format.

> 
> > + * Type:          11  1101  011   n<2:0>  011:n<3>
> > + * n: 0-3
> 
> My Arm ARM (DDI 0487E.a) says n can be in the [0, 15] range, despite there
> being only 4 architected counters ATM. Shouldn't matter too much now, but
> when more architected counters are added we'll have to assert 'n' against
> something (some revision #?).
> 

You are correct, that interval for the values of n should change. I
probably mapped my brain to the current architected counters. 

But the way I've defined SYS_AMEVCNTR0_EL0 will allow to access the full
range of 16 counters, for future versions of the AMU. I am hoping that
we won't have to directly use information in the feature register in
regards to the version of AMU. These first 4 architected counters should
be present in all future versions, and later we can use information in
AMCGCR_EL0 to get the number of architected counters (n) and
AMEVTYPER0<n>_EL0 to find out the type. The same logic would apply to
the auxiliary counters.

> > + *
> > + * Group 1 of activity monitors (auxiliary):
> > + *                op0 CRn   op1   op2     CRm
> > + * Counter:       11  1101  011   n<2:0>  110:n<3>
> > + * Type:          11  1101  011   n<2:0>  111:n<3>
> > + * n: 0-15
> > + */
> > +
> > +#define SYS_AMEVCNTR0_EL0(n)            SYS_AM_EL0(4 + ((n) >> 3), (n) & 0x7)
>                                                                           /^^^^
> If you want to be fancy, you could use GENMASK(2, 0) --------------------/
> 

I'll be fancy!

> > +#define SYS_AMEVTYPE0_EL0(n)            SYS_AM_EL0(6 + ((n) >> 3), (n) & 0x7)
> > +#define SYS_AMEVCNTR1_EL0(n)            SYS_AM_EL0(12 + ((n) >> 3), (n) & 0x7)
> > +#define SYS_AMEVTYPE1_EL0(n)            SYS_AM_EL0(14 + ((n) >> 3), (n) & 0x7)
> > +
> > +/* V1: Fixed (architecturally defined) activity monitors */
> > +#define SYS_AMEVCNTR0_CORE_EL0          SYS_AMEVCNTR0_EL0(0)
> > +#define SYS_AMEVCNTR0_CONST_EL0         SYS_AMEVCNTR0_EL0(1)
> > +#define SYS_AMEVCNTR0_INST_RET_EL0      SYS_AMEVCNTR0_EL0(2)
> > +#define SYS_AMEVCNTR0_MEM_STALL         SYS_AMEVCNTR0_EL0(3)
> > +
> >  #define SYS_CNTFRQ_EL0			sys_reg(3, 3, 14, 0, 0)
> >  
> >  #define SYS_CNTP_TVAL_EL0		sys_reg(3, 3, 14, 2, 0)
> 
> > @@ -1150,6 +1152,59 @@ static bool has_hw_dbm(const struct arm64_cpu_capabilities *cap,
> >  
> >  #endif
> >  
> > +#ifdef CONFIG_ARM64_AMU_EXTN
> > +
> > +/*
> > + * This per cpu variable only signals that the CPU implementation supports
> > + * the Activity Monitors Unit (AMU) but does not provide information
> > + * regarding all the events that it supports.
> > + * When this amu_feat per CPU variable is true, the user of this feature
> > + * can only rely on the presence of the 4 fixed counters. But this does
> > + * not guarantee that the counters are enabled or access to these counters
> > + * is provided by code executed at higher exception levels.
> > + *
> > + * Also, to ensure the safe use of this per_cpu variable, the following
> > + * accessor is defined to allow a read of amu_feat for the current cpu only
> > + * from the current cpu.
> > + *  - cpu_has_amu_feat()
> > + */
> > +static DEFINE_PER_CPU_READ_MOSTLY(u8, amu_feat);
> > +
> 
> Why not bool?
> 

I've changed it from bool after a sparse warning about expression using
sizeof(bool) and found this is due to sizeof(bool) being compiler
dependent. It does not change anything but I thought it might be a good
idea to define it as 8-bit unsigned and rely on fixed size.

Thank you for the review,
Ionela.

> > +inline bool cpu_has_amu_feat(void)
> > +{
> > +	return !!this_cpu_read(amu_feat);
> > +}
> > +
> > +static void cpu_amu_enable(struct arm64_cpu_capabilities const *cap)
> > +{
> > +	if (has_cpuid_feature(cap, SCOPE_LOCAL_CPU)) {
> > +		pr_info("detected CPU%d: Activity Monitors Unit (AMU)\n",
> > +			smp_processor_id());
> > +		this_cpu_write(amu_feat, 1);
> > +	}
> > +}

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