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.
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:-
- 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.
- 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.
Leo