Re: [PATCH 2/6] perf cs-etm: Initialise architecture based on TRCIDR1

From: Leo Yan
Date: Mon Aug 02 2021 - 11:04:10 EST

Hi Mike,

On Mon, Aug 02, 2021 at 03:04:14PM +0100, Mike Leach wrote:


> > > > +#define TRCIDR1_TRCARCHMIN_SHIFT 4
> > > > +#define TRCIDR1_TRCARCHMIN_MASK GENMASK(7, 4)
> > > > +static enum _ocsd_arch_version cs_etm_decoder__get_arch_ver(u32 reg_idr1)
> > > > +{
> > > > + /*
> > > > + * If the ETM trace minor version is 4 or more then we can assume
> > > > + * the architecture is ARCH_AA64 rather than just V8
> > > > + */
> > > > + return TRCIDR1_TRCARCHMIN(reg_idr1) >= 4 ? ARCH_AA64 : ARCH_V8;
> > > > +}
> > >
> > > This is true for ETM4.x & ETE 1.x (arch 5.x) but not ETM 3.x
> > > Probably need to beef up this comment or the function name to emphasise this.
> >
> > Yeah, I think it's good to change the function name. Eventually, this
> > function should only be used for ETM4.x and ETE.
> >
> > Another minor comment is: can we refine the arch version number, e.g.
> > change the OpenCSD's macro "ARCH_AA64" to "ARCH_V8R4", (or
> > "ARCH_V8R3_AA64"), this can give more clear clue what's the ETM version.
> >
> The purpose of these macros is to inform the decoder of the
> architecture of the PE - not the version of the ETM.
> These OpenCSD macros are defined by the library headers
> (ocsd_if_types.h) and not the perf headers.
> These have been published as the API / ABI for OpenCSD and as such
> changing them affects all OpenCSD clients, not just perf.

I understand these macros are defined in OpenCSD lib as APIs, since I
saw these macros have not been widely used in perf tool (e.g.
ARCH_AA64), so this is why I think it's good to take chance to refine
the naming conventions.

> This PE architecture version is used along with the core profile to
> determine which instructions are valid waypoint instructions to
> associate with atom elements when walking the program image during
> trace decode.
> From v8.3 onwards we moved away from filtering on specific
> architecture versions. This was due to two factors:-
> 1. The architectural rules now allow architectural features for one
> increment e.g. Arch 8.4, to be backported into the previous increment
> - e,g, 8.3, which made this filtering more difficult to track.
> 2. After discussion with the PE architects it was clear that
> instructions in a later architect version would not re-use older
> opcodes from a previous one and be nop / invalid in the earlier
> architectures. (certainly in the scope of AA64). Therefore
> the policy in the decoder is to check for all the instructions we know
> about for the latest version of architecture, even if we could be
> decoding an earlier architecture version. This means we may check for
> a few more opcodes than necessary for earlier version of the
> architecture, but the overall decode is more robust and easier to
> maintain.
> Therefore for any AA64 core beyond v8.3 - it is safe to use the
> ARCH_AA64 PE architecture version and the decoder will handle it.

I have no objection for current approach; but two things can cause
confusions and it might be difficult for maintenance:

- The first thing is now we base on the bit fields TRCIDR1::TRCARCHMIN
to decide the PE architecture version. In the ETMv4 spec,
TRCIDR1::TRCARCHMIN is defined as the trace unit minor version,
so essentially it's a minor version number for tracer (ETM) but not
the PE architecture number. But now we are using it to decide the
PE architecture number (8.3, 8.4, etc...).

- The second thing is the macros' naming convention.
E.g. "AA64" gives me an impression it is a general naming "Arm Arch 64"
for all Arm 64-bit CPUs, it's something like an abbreviation for
"aarch64"; so seems to me it doesn't show any meaningful info for PE's
architecture version number. This is why I proposed to use more
explict macro definition for architectures (e.g. ARCH_V8R3, ARCH_V8R4,
ARCH_V9R0, etc).

If we really want to use ARCH_AA64, it's better to give some comments in
the code.

Thanks a lot for shared the background info.