Re: [Qemu-devel] [RFC v1 PATCH 2/3] cpus: Convert cpu_index into a bitmap

[Eduardo Habkost]

On Fri, May 08, 2015 at 03:21:35PM +0530, Bharata B Rao wrote:
> Currently CPUState.cpu_index is monotonically increasing and a newly
> created CPU always gets the next higher index. The next available
> index is calculated by counting the existing number of CPUs. This is
> fine as long as we only add CPUs, but there are architectures which
> are starting to support CPU removal too. For an architecture like PowerPC
> which derives its CPU identifier (device tree ID) from cpu_index, the
> existing logic of generating cpu_index values causes problems.
> 
> With the currently proposed method of handling vCPU removal by parking
> the vCPU fd in QEMU
> (Ref: http://lists.gnu.org/archive/html/qemu-devel/2015-02/msg02604.html),
> generating cpu_index this way will not work for PowerPC.
> 
> This patch changes the way cpu_index is handed out by maintaining
> a bit map of the CPUs that tracks both addition and removal of CPUs.
> 
> The CPU bitmap allocation logic is part of cpu_exec_init() which is
> called by instance_init routines of various CPU targets. This patch
> also adds corresponding instance_finalize routine if needed for these
> CPU targets so that CPU can be marked free when it is removed.
> 
> Signed-off-by: Bharata B Rao <address@hidden>
> ---
>  exec.c                      | 37 ++++++++++++++++++++++++++++++++++---
>  include/qom/cpu.h           |  8 ++++++++
>  target-alpha/cpu.c          |  6 ++++++
>  target-arm/cpu.c            |  1 +
>  target-cris/cpu.c           |  6 ++++++
>  target-i386/cpu.c           |  6 ++++++
>  target-lm32/cpu.c           |  6 ++++++
>  target-m68k/cpu.c           |  6 ++++++
>  target-microblaze/cpu.c     |  6 ++++++
>  target-mips/cpu.c           |  6 ++++++
>  target-moxie/cpu.c          |  6 ++++++
>  target-openrisc/cpu.c       |  6 ++++++
>  target-ppc/translate_init.c |  6 ++++++
>  target-s390x/cpu.c          |  1 +
>  target-sh4/cpu.c            |  6 ++++++
>  target-sparc/cpu.c          |  1 +
>  target-tricore/cpu.c        |  5 +++++
>  target-unicore32/cpu.c      |  6 ++++++
>  target-xtensa/cpu.c         |  6 ++++++
>  19 files changed, 128 insertions(+), 3 deletions(-)

Why not simply call cpu_exec_exit() on generic CPU::instance_finalize,
to avoid forcing every architecture to call it manually? Calling
cpu_exec_exit() twice would be harmless, anyway.

(It would just need an additional check to make sure the bit will be
cleared only if cpu_exec_init() was really called and cpu_index was
properly set.)

-- 
Eduardo