Messages in this thread | | | From | Mike Leach <> | Date | Mon, 2 Aug 2021 16:43:59 +0100 | Subject | Re: [PATCH 2/6] perf cs-etm: Initialise architecture based on TRCIDR1 |
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Hi Leo,
On Mon, 2 Aug 2021 at 16:04, Leo Yan <leo.yan@linaro.org> wrote: > > 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) > > > > > +#define TRCIDR1_TRCARCHMIN(x) (((x) & TRCIDR1_TRCARCHMIN_MASK) >> TRCIDR1_TRCARCHMIN_SHIFT) > > > > > +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. >
The macros are used in other tools - so changing now affects those too. Not something I am prepared to do without good reason.
> > 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...). >
This is a slight weakness in the implementation of perf. Ideally one does need to establish the architecture version of the PE - but perf /cs-etm is using an assumption regarding the profile and version of the core, according to the ETM / ETE versiom. That said - the ETM / ETE version numbers do have a strong relationship with PE architecture version numbers, so this assumption holds for the current supported devices.
> - 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). >
For modern cores it is sufficient for the decoder to know the profile and that it is aarch 64 - so yes the macro is simply saying this a general AA64 core. The macros for earlier versions are a little more specific as certain filtering is used according to the version of the PE.
ARCH_V8R4, ARCH_V9R0 etc would have no significance to the decoder and would not be useful. If we get to the stage where we need more specific PE architecture versions - then these can be added as required. Using the ARCH_AA64 macro means that we do not have to update the API for every version update of the architecture, and there are no changes required to the perf / cs-etm handling.
> If we really want to use ARCH_AA64, it's better to give some comments in > the code. >
There are comments in the OpenCSD headers, though additional ones in the perf / cs-etm handling soruce code could be added.
Regards
Mike
> Thanks a lot for shared the background info. > > Leo
-- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK
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