[Qemu-devel] [RFC 00/20] Do away with TB retranslation

[Richard Henderson <rth@twiddle.net>]

I've been looking at this problem off and on for the last week or so,
prompted by the sparc performance work.  Although I havn't been able
to get a proper sparc64 guest install working, I see the exact same
problem with a mips guest.

On alpha or x86, which seem to perform well, perf numbers for the
executable have about 30% of the execution time spent in cpu_exec.
For mips, on the other hand, we spend about 30% of the time in
routines related to tcg (re-)translation.

Aurelien has a patch in his own branches that attempts to mitigate this
on mips by shadow caching more tlb entries.  While this does improve
performace a bit, it employs a linear search through a large buffer,
with the effect of 30-ish % perf numbers for r4k_map_address.

(One could probably improve things by hashing the data in that array,
rather than a linear search, but...)

In the past we've talked about getting rid of retranslation entirely.
It's clever, but it certainly has its share of problems.  I gave it
a go this weekend.

The following isn't quite right.  It fails to boot on sparc even with
our tiny test kernel.  It also triggers an abort on mips, eventually.
But it's able to get all the way through to a prompt, and in the 
process I can see that perf results are quite different -- much more
like results I see for alpha.

Thoughts on the approach?


r~


Richard Henderson (20):
  tcg: Rename debug_insn_start to insn_start
  target-*: Unconditionally emit tcg_gen_insn_start
  tcg: Allow extra data to be attached to insn_start
  target-arm: Add condexec state to insn_start
  target-i386: Add cc_op state to insn_start
  target-mips: Add delayed branch state to insn_start
  target-s390x: Add cc_op state to insn_start
  target-sh4: Add flags state to insn_start
  target-cris: Mirror gen_opc_pc into insn_start
  target-sparc: Tidy gen_branch_a interface
  target-sparc: Split out gen_branch_n
  target-sparc: Remove gen_opc_jump_pc
  target-sparc: Add npc state to insn_start
  tcg: Merge cpu_gen_code into tb_gen_code
  target-*: Drop cpu_gen_code define
  tcg: Add TCG_MAX_INSNS
  tcg: Pass data argument to restore_state_to_opc
  tcg: Save insn data and use it in cpu_restore_state_from_tb
  tcg: Remove gen_intermediate_code_pc
  tcg: Remove tcg_gen_code_search_pc

 include/exec/exec-all.h       |   6 +-
 target-alpha/cpu.h            |   1 -
 target-alpha/translate.c      |  55 +++-------
 target-arm/cpu.h              |   2 +-
 target-arm/translate-a64.c    |  39 ++-----
 target-arm/translate.c        |  75 ++++---------
 target-arm/translate.h        |   8 +-
 target-cris/cpu.h             |   1 -
 target-cris/translate.c       |  64 +++---------
 target-cris/translate_v10.c   |   3 -
 target-i386/cpu.h             |   2 +-
 target-i386/translate.c       |  86 ++++-----------
 target-lm32/cpu.h             |   1 -
 target-lm32/translate.c       |  55 ++--------
 target-m68k/cpu.h             |   1 -
 target-m68k/translate.c       |  64 +++---------
 target-microblaze/cpu.h       |   1 -
 target-microblaze/translate.c |  56 +++-------
 target-mips/cpu.h             |   2 +-
 target-mips/translate.c       |  73 ++++---------
 target-moxie/cpu.h            |   1 -
 target-moxie/translate.c      |  65 ++++--------
 target-openrisc/cpu.h         |   1 -
 target-openrisc/translate.c   |  54 ++--------
 target-ppc/cpu.h              |   1 -
 target-ppc/translate.c        |  56 +++-------
 target-s390x/cpu.h            |   2 +-
 target-s390x/translate.c      |  61 +++--------
 target-sh4/cpu.h              |   2 +-
 target-sh4/translate.c        |  71 ++++---------
 target-sparc/cpu.h            |   2 +-
 target-sparc/translate.c      | 189 ++++++++++++++-------------------
 target-tricore/translate.c    |  53 ++++------
 target-unicore32/translate.c  |  57 +++-------
 target-xtensa/cpu.h           |   1 -
 target-xtensa/translate.c     |  52 ++-------
 tcg/tcg-op.h                  |  52 +++++++--
 tcg/tcg-opc.h                 |   4 +-
 tcg/tcg.c                     |  96 ++++++++---------
 tcg/tcg.h                     |  14 ++-
 tci.c                         |   9 --
 translate-all.c               | 237 ++++++++++++++++++++++++------------------
 42 files changed, 578 insertions(+), 1097 deletions(-)

-- 
2.4.3