Re: [PATCH v2] perf: Support for Arm A32/T32 instruction sets in CoreSight trace

[Mathieu Poirier]

On Fri, 31 Aug 2018 at 08:43, Kim Phillips <kim.phillips@xxxxxxx> wrote:
>
> On Fri, 31 Aug 2018 14:42:00 +0100
> Robert Walker <robert.walker@xxxxxxx> wrote:
>
> > Generally, I agree with you about breaking backward compatibility, but
> > in this case I don't think there is an actual problem.  As I understand
>
> I consider it a serious problem.
>
> > it, you're worried that perf will break for people who are using an
> > older version (0.8.x) of the OpenCSD library for CoreSight trace decode
> > and this patch updates the requirement to a newer version (0.9.x) to
> > enable support for trace of 32-bit applications.
>
> My problem is: every time a new feature is added, it shouldn't
> force base CoreSight decode functionality to require a new version of
> the library.
>
> My second problem is: this patch implements a build-time requirement,
> which is insufficient for running on machines with incompatible
> versions of the library.

While it is not realistic to expect eternal backward compatibility for
core features, I think in this case we can do better.

Looking at the code in the patch it seems possible to implement the
new functionality in functions that can be enabled based on the
library version we compile against.  If the library version is not
high enough, the functions simply get stubbed out and the feature
isn't available.

A prerequisite step would be to move openCSD's
"decoder/include/ocsd_if_version.h" under
"decoder/include/opencsd/ocsd_if_version.h" in order to make symbols
OCSD_VER_{ MAJOR | MINOW | PATH } accessible but that's not a big
deal.

Please try this approach first - we can consider more drastic measures
if that doesn't work.

>
> > There are only a few (4/5?) targets around with working support for
> > CoreSight trace (and of these only Juno is the only platform with a
> > devicetree in the mainline kernel), so only a few users of perf for Arm
> > trace decode - most of these are people working those directly involved
> > with Arm & Linaro or will be reading the coresight mailing list.  Anyone
>
> Great, then share this feature with them, but don't send a patch to
> break upstream, which, in turn, goes back to many things downstream
> (future distro releases on newer targets, etc.).
>
> > working with OpenCSD will have got it from github and compiled it
> > themselves, so they can update and build a new version.  It's only been
>
> No.  Updating the library - no matter where one gets it from - shouldn't
> have to be necessary to avoid perf build regressions.
>
> > packaged for debian so far and testing already has the 0.9.x version
> > (the 0.8.x version was only in debian for 8 days before being replaced
> > by 0.9.x).
>
> What Debian does is immaterial to upstream submissions.
>
> How is Debian testing the library, btw?  Functionality that worked in
> 0.8 will fail in 0.9 AFAICT.
>
> > It would be possible to add conditional compilation flags to support
> > compiling with 0.8.x, but I feel this would add too much mess to the
> > code and I'd need some help in figuring out perf's feature detection
> > system to generate the flags.
>
> No, we need run-time compatibility.  Build-time compatibility does not
> satisfy the run-time requirements in this case.
>
> >  Given the likely small number of people
> > affected and the easy upgrade path, I don't think this is worth it.
>
> This is an upstream submission, and I wouldn't like for a single person
> to ever have to experience such silently deceitful bugs.
>
> For now, share the new feature in a personal git tree, for those that
> need it.
>
> Meanwhile, the library needs to be fixed with a run-time version
> compatibility API ASAP.
>
> Thanks,
>
> Kim
>
> > On 29/08/18 17:32, Kim Phillips wrote:
> > > On Wed, 29 Aug 2018 15:34:16 +0100
> > > Robert Walker <robert.walker@xxxxxxx> wrote:
> > >
> > >> Hi Kim,
> > > Hi Robert,
> > >
> > >> On 29/08/18 14:49, Kim Phillips wrote:
> > >>> On Wed, 29 Aug 2018 10:44:23 +0100
> > >>> Robert Walker <robert.walker@xxxxxxx> wrote:
> > >>>
> > >>>> This patch adds support for generating instruction samples from trace of
> > >>>> AArch32 programs using the A32 and T32 instruction sets.
> > >>>>
> > >>>> T32 has variable 2 or 4 byte instruction size, so the conversion between
> > >>>> addresses and instruction counts requires extra information from the trace
> > >>>> decoder, requiring version 0.9.1 of OpenCSD.  A check for the new struct
> > >>>> member has been added to the feature check for OpenCSD.
> > >>>>
> > >>>> Signed-off-by: Robert Walker <robert.walker@xxxxxxx>
> > >>>> ---
> > >>> ...
> > >>>> +++ b/tools/build/feature/test-libopencsd.c
> > >>>> @@ -3,6 +3,13 @@
> > >>>>
> > >>>>    int main(void)
> > >>>>    {
> > >>>> +        /*
> > >>>> +         * Requires ocsd_generic_trace_elem.num_instr_range introduced in
> > >>>> +         * OpenCSD 0.9
> > >>> 0.9 != 0.9.1 in the above commit text: which is it?
> > >> I'll change it to 0.9.1 if there's another version of the patch (it was
> > >> introduced in 0.9, but 0.9.1 has a necessary bug fix)
> > >>>> +         */
> > >>>> +        ocsd_generic_trace_elem elem;
> > >>>> +        (void)elem.num_instr_range;
> > >>>> +
> > >>> This breaks building against older versions of OpenCSD, right?
> > >>>
> > >>>>          (void)ocsd_get_version();
> > >>> Why don't we maintain building against older versions of the library,
> > >>> and use the version information to make the decision on whether to use
> > >>> the new feature being introduced in this patch?
> > >> The intention is to fail the feature detection check if the older
> > >> version is installed - perf will still compile, but without the
> > >> CoreSight trace support.
> > > It should still compile, and with CoreSight trace support, just
> > > not support for A32/T32 instruction sets.  The user shouldn't be denied
> > > CoreSight trace support if they don't care for A32/T32 ISA support.
> > >
> > >> OpenCSD is still in development, so new features like this are being
> > >> added and it would add a lot of #ifdef mess to the perf code to continue
> > >> to support any machines with the old library version installed - there
> > > Even adding #ifdefs, that won't survive taking one perf executable
> > > built on one machine and running it on another machine with a different
> > > version of the OpenCSD library: it'll break inconspicuously, not
> > > gracefully!
> >
> > perf has a lot of other shared library dependencies (ELF , unwind
> > libraries etc), so moving builds between systems is already fragile.
> >
> > > There needs to be a run-time means of working with older versions of
> > > the library.
> > >
> > > Consider checking the sizeof some of the structs?  IIRC, some of the
> > > structs sizes changed in the library.  See e.g., the 'size' field of
> > > perf_event_attr:
> > >
> > > size
> > >       The size of the perf_event_attr structure for forward/backward
> > >       compatibility.  Set this using sizeof(struct perf_event_attr)
> > >       to allow the kernel to see the struct size at the time
> > >       of compilation.
> > >
> > > or, likely better, the 'version' and 'compat_version' of the
> > > perf_event_mmap_page structure:
> > >
> > >             struct perf_event_mmap_page {
> > >                 __u32 version;        /* version number of this structure */
> > >                 __u32 compat_version; /* lowest version this is compat with */
> > >            ...
> > >
> > >> will only be a handful of machines affected and it's trivial to upgrade
> > >> them (the new Debian packages are available).
> > > This is upstream linux, so I don't know how you know only a 'handful'
> > > of machines affected, and I wouldn't assume everyone's using Debian.
> > >
> > > For one, I'd hate to see a single user affected if it isn't necessary,
> > > as is in this case - not everyone wants A32/T32 ISA support, and
> > > library compatibility needn't be broken.
> > >
> > > This 'screw compatibility' mentality needs to be dropped *now* if
> > > CoreSight support is to have a successful future.
> > >
> > > Otherwise, I suggest keeping this feature in downstream trees for the
> > > 'handful', until the library and perf code are rewritten in a state
> > > where they properly interoperate, and do not break each other.
> > >
> > >> How long would we
> > >> continue to support such an older version?
> > > What do you mean such an older version?  The project's v0.9.0 commit
> > > was on 20 June 2018, the one that's usable - v0.9.1 - has a July 27
> > > 2018 commit date!  One month is *not* *old*!
> > I mean the 0.8.x version as the old version.
> > >>    I also don't see any
> > >> precedent for supporting multiple dependent library versions in perf.
> > > That's because perf doesn't have a precedent on depending on libraries
> > > that flat-out break their own users compatibility across versions ;)
> > This patch picks up a new feature that's been added - I notice the
> > feature detection checks for other libraries check a number of features
> > and emit warnings about required versions.
> >
> > > Thanks,
> > >
> > > Kim
> >
> > Regards
> >
> > Rob
> >
> >