Re: [Qemu-devel] [PATCH v8 0/5] hypertrace: Lightweight guest-to-QEMU trace channel

[Stefan Hajnoczi]

On Sun, Jul 30, 2017 at 05:08:18PM +0300, Lluís Vilanova wrote:
> The hypertrace channel allows guest code to emit events in QEMU (the host) 
> using
> its tracing infrastructure (see "docs/trace.txt"). This works in both 'system'
> and 'user' modes, is architecture-agnostic and introduces minimal noise on the
> guest.
> 
> See first commit for a full description, use-cases and an example.
> 
> Signed-off-by: Lluís Vilanova <address@hidden>
> ---
> 
> Changes in v8
> =============
> 
> * Do not use 'seq' when there's no extra hypertrace arguments (BSD behaves
>   differently for "seq 0").
> * Fix compilation for bsd-user.

Hi Lluís,
Any changes regarding the fundamental approach since September 2016?

Back then I had the following concerns about hypertrace for full-system
virtualization:

Going to QEMU for every guest trace event has high overhead under
virtualization.  The alternative approach is recording separate traces
and merging them for analysis.  This is possible with trace-cmd [1] and
LTTng [2].

Merging traces eliminates the performance bottleneck and does not
require new paravirt interfaces or guest tracing libraries.  I think it
it would be a distraction to support hypertrace for the virtualization
use case because it fundamentally has a high overhead.

I see promise in using hypertrace for TCG because it is low-overhead
when running inside the QEMU process.  I'll review the patches again
with this in mind and not focus on virtualization.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg00887.html
[2] 
http://archive.eclipse.org/tracecompass/doc/stable/org.eclipse.tracecompass.doc.user/Virtual-Machine-Analysis.html
    and also generic trace synchronization
    
http://archive.eclipse.org/tracecompass/doc/stable/org.eclipse.tracecompass.doc.user/Trace-synchronization.html#Trace_synchronization

Stefan

> Changes in v7
> =============
> 
> * Use 'expr' instead of assuming 'bash' when generating the "emit.c" file.
> * Restore generation of trace-events-all.
> 
> 
> Changes in v6
> =============
> 
> * Fix compilation errors.
> 
> 
> Changes in v5
> =============
> 
> * Rebase on 5a477a7806.
> * Fix typo in "bsd-user/main.c" [Stephan Hajnoczi].
> * Replace abort() with exit() in command-line errors [Stephan Hajnoczi].
> * Fix alignment of data and control channels [Stephan Hajnoczi].
> * Fix signal reflection in user-mode (SIGINT, SIGABRT, SIGSEGV) [Stephan 
> Hajnoczi].
> * Clarify semantics of hypertrace_guest_mmap_check() [Stephan Hajnoczi].
> * Use uintptr_t instead of unsigned long in SEGV handler [Stephan Hajnoczi].
> * Emit hypertrace's event with host-endian arguments [Stephan Hajnoczi].
> * Enable true concurrency between user-mode guest threads by using a spearate 
> control channel page per client [Stephan Hajnoczi].
> * Remove unused PAGE_SIZE define [Stephan Hajnoczi].
> * Submit linux kernel API module separately to Linux upstream [Stephan 
> Hajnoczi].
> * Assume guest code events are always enabled.
> 
> 
> Changes in v4
> =============
> 
> * Fix typo in stap script example.
> * Fix compilation instructions in doc/hypertrace.txt.
> * Rebase on 0737f32daf.
> 
> 
> Changes in v3
> =============
> 
> * Rebase on 4a58f35.
> * Remove debugging printf's.
> * Fix style issues identified by checkpatch.
> * Fix control channel mapping in guest linux module.
> * Add a short event description in "trace-events".
> * Polish documentation in 1st patch.
> 
> 
> Changes in v2
> =============
> 
> * Remove unnecessary casts for g2h() [Eric Blake].
> * Use perror() [Eric Blake].
> * Avoid expansions in application example [Eric Blake].
> * Add copyright in document "hypertrace.txt" [Eric Blake].
> * Make the user-mode hypertrace invocations thread-safe [Stefan Hajnoczi].
> * Split dynamic hypertrace configuration into a separate "config" channel.
> 
> Lluís Vilanova (5):
>       hypertrace: Add documentation
>       hypertrace: Add tracing event "guest_hypertrace"
>       hypertrace: [*-user] Add QEMU-side proxy to "guest_hypertrace" event
>       hypertrace: [softmmu] Add QEMU-side proxy to "guest_hypertrace" event
>       hypertrace: Add guest-side user-level library
> 
> 
>  Makefile                           |   11 +
>  Makefile.objs                      |    6 +
>  bsd-user/main.c                    |   17 +
>  bsd-user/mmap.c                    |   15 +
>  bsd-user/qemu.h                    |    3 
>  bsd-user/syscall.c                 |   34 ++-
>  configure                          |   36 +++
>  docs/devel/tracing.txt             |    3 
>  docs/hypertrace.txt                |  225 ++++++++++++++++++++
>  hypertrace/Makefile.objs           |   25 ++
>  hypertrace/common.c                |   55 +++++
>  hypertrace/common.h                |   25 ++
>  hypertrace/guest/Makefile          |   30 +++
>  hypertrace/guest/common.c          |  301 ++++++++++++++++++++++++++
>  hypertrace/guest/qemu-hypertrace.h |   80 +++++++
>  hypertrace/softmmu.c               |  237 +++++++++++++++++++++
>  hypertrace/user.c                  |  415 
> ++++++++++++++++++++++++++++++++++++
>  hypertrace/user.h                  |   71 ++++++
>  include/hw/pci/pci.h               |    2 
>  include/qom/cpu.h                  |    4 
>  linux-user/main.c                  |   19 ++
>  linux-user/mmap.c                  |   16 +
>  linux-user/qemu.h                  |    3 
>  linux-user/signal.c                |   12 +
>  linux-user/syscall.c               |   31 ++-
>  rules.mak                          |    2 
>  trace-events                       |   11 +
>  27 files changed, 1660 insertions(+), 29 deletions(-)
>  create mode 100644 docs/hypertrace.txt
>  create mode 100644 hypertrace/Makefile.objs
>  create mode 100644 hypertrace/common.c
>  create mode 100644 hypertrace/common.h
>  create mode 100644 hypertrace/guest/Makefile
>  create mode 100644 hypertrace/guest/common.c
>  create mode 100644 hypertrace/guest/qemu-hypertrace.h
>  create mode 100644 hypertrace/softmmu.c
>  create mode 100644 hypertrace/user.c
>  create mode 100644 hypertrace/user.h
> 
> 
> To: address@hidden
> Cc: Stefan Hajnoczi <address@hidden>
> Cc: Eric Blake <address@hidden>
> Cc: Luiz Capitulino <address@hidden>
> Cc: Daniel P Berrange <address@hidden>

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