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

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Date:   Tue, 28 Jan 2020 11:00:07 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     catalin.marinas@....com, will@...nel.org, mark.rutland@....com,
        maz@...nel.org, suzuki.poulose@....com, sudeep.holla@....com,
        dietmar.eggemann@....com, peterz@...radead.org, mingo@...hat.com,
        ggherdovich@...e.cz, vincent.guittot@...aro.org,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] arm64: add support for the AMU extension v1

Hi Valentin,

On Friday 24 Jan 2020 at 12:00:25 (+0000), Valentin Schneider wrote:
> On 23/01/2020 18:32, Ionela Voinescu wrote:
> [...]
> > 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.
> > 
> 
> Good, I think that's all we'll really need. I've not gone through the whole
> series (yet!) so I might've missed AMCGCR being used.
>

No, it's not used later in the patches either, specifically because
this is version 1 and we should be able to rely on these first 4
architected counters for all future versions of the AMU implementation.

> >>> @@ -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.
> > 
> 
> I believe conveying the intent (a truth value) is more important than the
> underlying storage size in this case. It mostly matters when dealing with
> aggregates, but here it's just a free-standing variable.
> 
> We already have a few per-CPU boolean variables in arm64/kernel/fpsimd.c
> and the commits aren't even a year old, so I'd go for ignoring sparse this
> time around.
>

Will do!

Thanks,
Ionela.