Hi, On Thu, 4 Feb 2021 at 11:08, Suzuki K Poulose <suzuki.poul...@arm.com> wrote: > > On 2/4/21 4:09 AM, Leo Yan wrote: > > Hi Mike, > > > > On Wed, Feb 03, 2021 at 05:39:54PM +0000, Mike Leach wrote: > > > > [...] > > > >>> +2.2) Tracing PID > >>> + > >>> +When the kernel is running at EL2 with Virtualization Host Extensions > >>> (VHE), > >>> +perf records CONTEXTIDR_EL2 in the trace data and can be used as PID when > >>> +decoding; and if the kernel is running at EL1 with nVHE, CONTEXTIDR_EL1 > >>> is > >>> +traced for PID. > >>> + > >> > >> Would this introductory paragraph be better if is explained where the > >> kernel stores the PID for the different levels, then we logically move > >> on to how to trace this in perf. > >> > >> e.g:- > >> > >> "The lernel can be built to write the PID value into the PE ContextID > >> registers. > >> For a kernel running at EL1, the PID is stored in CONTEXTIDR_EL1. > >> A PE may implement ARM Virtualisation Host Extensions (VHE), were the > >> kernel can run at EL2 as a virtualisation host. > >> In this case the PID value is stored in CONTEXTIDR_EL2. > >> perf provides PMU options which program the ETM to insert these values > >> into the trace data." > > > > Will in next spin; thanks a lot for writing up! > > > >>> +To support tracing PID for the kernel runs at different exception levels, > >>> +the PMU formats are defined as follow: > >>> + > >>> + "contextid1": Available on both EL1 kernel and EL2 kernel. When the > >>> + kernel is running at EL1, "contextid1" enables the PID > >>> + tracing; when the kernel is running at EL2, this enables > >>> + tracing the PID of guest applications. > >>> + > >>> + "contextid2": Only usable when the kernel is running at EL2. When > >>> + selected, enables PID tracing on EL2 kernel. > >>> + > >>> + "contextid": Will be an alias for the option that enables PID > >>> + tracing. I.e, > >>> + contextid == contextid1, on EL1 kernel. > >>> + contextid == contextid2, on EL2 kernel. > >>> + > >>> +The perf tool automatically sets corresponding bit for the "contextid" > >>> config, > >>> +therefore, the user doesn't have to bother which EL the kernel is > >>> running. > >>> + > >>> + i.e, perf record -e cs_etm/contextid/u -- uname > >>> + or perf record -e cs_etm//u -- uname > >>> + > >>> +will always do the "PID" tracing, independent of the kernel EL. > >>> + > >> > >> This is telling me that both cs_etm// and cs_etm/contextid/ have the > >> same effect - trace PID. Is this correct? > > > > Just to make this clear, this is not a side effect of the patch.
Which is fine - but the documentation should accurately reflect what is happening on the system. This is a new paragraph about the PID tracing or otherwise, Even if some of the effects pre-date this patch, they have to be accurately communicated. I am also reading the new paragraph in the context of the rest of the coresight.rst document - which is a user level document explaining the basic operation of the coresight system and tools. This document mentions no other perf command line parameters relevant to coresight other than the @sink option.It actually calls out to the OpenCSD docs to provide further information. > The perf > tool driver automatically adds the "contextid" tracing and timestamp for > "system wide" and process bound events, as they traces get mixed into > the single sink. So these options are added implicitly by the perf tool > to make the decoding easier. > That's fine - I have no problem with contextID trace enabled by default. Context ID is relatively low overhead - and only emitted at start of trace / context changes. But the explanation of the parameters currently reads as though they always have an effect - and not putting them in there will omit the effect - unless you spot the very subtle line at the end. The user does not need to know about parameters that have no effect! Perhaps a better approach would be to explain the above - an explicit statement that "perf will always enable PID/ contextID tracing at the relevant EL - but for EL2 it is possible to make specific adjustments using parameters......." Cheers Mike > > Correct. > > > >> If so, then contextid, contextid1 and contextid2 have no effect except > >> in specific EL2 circumstances. > > These are required when perf tool may not automatically request them. > With this series the EL2 is on par with the EL1, where we get the PID > automatcially in the trace. > > And as you rightly said, contextid1, contextid2 are for EL2 specific > usage. > > Cheers > Suzuki > > > > > Yes, exactly. > > > > Thanks, > > Leo > > > >>> +When the kernel is running at EL2 with VHE, if user wants to trace both > >>> the > >>> +PIDs for both host and guest, the two configs "contextid1" and > >>> "contextid2" > >>> +can be set at the same time: > >>> + > >>> + perf record -e cs_etm/contextid1,contextid2/u -- uname > >>> + > >>> > >> > >> > >> Regards > >> > >> Mike > >> > >> > >>> Generating coverage files for Feedback Directed Optimization: AutoFDO > >>> --------------------------------------------------------------------- > >>> -- > >>> 2.25.1 > >>> > >> > >> > >> -- > >> Mike Leach > >> Principal Engineer, ARM Ltd. > >> Manchester Design Centre. UK > -- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK
