linux-kernel - Re: [PATCH 03/23] perf: Add ability to attach user level registers dump to sample

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Date:	Wed, 27 Jun 2012 17:30:05 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Stephane Eranian <eranian@...gle.com>,
	Jiri Olsa <jolsa@...hat.com>, acme@...hat.com, mingo@...e.hu,
	paulus@...ba.org, cjashfor@...ux.vnet.ibm.com, gorcunov@...nvz.org,
	tzanussi@...il.com, mhiramat@...hat.com, robert.richter@....com,
	fche@...hat.com, linux-kernel@...r.kernel.org,
	masami.hiramatsu.pt@...achi.com, drepper@...il.com, asharma@...com,
	benjamin.redelings@...cent.org,
	"H. Peter Anvin" <hpa@...ux.intel.com>
Subject: Re: [PATCH 03/23] perf: Add ability to attach user level registers
 dump to sample

On Wed, Jun 27, 2012 at 05:25:29PM +0200, Peter Zijlstra wrote:
> On Wed, 2012-06-27 at 17:24 +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 27, 2012 at 05:13:44PM +0200, Stephane Eranian wrote:
> > > On Wed, Jun 27, 2012 at 5:11 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > > On Tue, Jun 19, 2012 at 05:47:54PM +0200, Jiri Olsa wrote:
> > > >> @@ -4061,6 +4105,19 @@ void perf_prepare_sample(struct perf_event_header *header,
> > > >>               }
> > > >>               header->size += size;
> > > >>       }
> > > >> +
> > > >> +     if (sample_type & PERF_SAMPLE_REGS_USER) {
> > > >> +             /* regs dump available bool */
> > > >> +             int size = sizeof(u64);
> > > >> +
> > > >> +             data->regs_user = perf_sample_regs_user(regs);
> > > >> +             if (data->regs_user) {
> > > >> +                     u64 mask = event->attr.sample_regs_user;
> > > >> +                     size += hweight64(mask) * sizeof(u64);
> > > >> +             }
> > > >> +
> > > >> +             header->size += size;
> > > >
> > > > We'll need to remove the 64 bits registers for compat tasks but other than
> > > > that, it looks ok.
> > > You cannot do this. You cannot remove register values from under the hood.
> > > The only way the user has to parse the sample is the sample_regs_users bitmask.
> > > You have to return 0 for those unexisting regs for compat tasks.
> > 
> > You mean fill unexisting reg values with 0? Yeah that works.
> 
> What does x32 look like? Is that still reported as a compat task? If so,
> we should record all registers and not 0 out anything.

To make it simple, we could always use the x86-64 registers mask interface for everyone:
native 32 bits kernel, compat 32 bits task, 64 bits everything. And we can't fill an x86-64
value because we are dealing with a native/compat 32 task we just fill out the requested value
with 0.

On post processing time, userspace can know if it's dealing with 32 bits task or not anyway, so
it knows what to skip and what is relevant.
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