On Fri, May 08, 2015 at 11:55:00AM -0300, Eduardo Habkost wrote: > 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 <bhar...@linux.vnet.ibm.com> > > --- > > 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.
Yes cpu_exec_exit() can be called from generic CPU::instance_finalize and it does appear harmless calling it twice but, Can there be a situation where cpu_index freed from the first cpu_exec_exit() call from ->unrealize() be allocated (to a different caller) again before the 2nd call for the same CPU from CPU::instance_finalize ? If yes, cpu_exec_exit() needs to be more intelligent than what it is currently is. > > (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.) If the situation I describe above can indeed happen, then cpu_exec_exit() needs to maintain state to safely fail the double free for the same CPU from the same caller. I think touching all archs and adding instance_finalize would be much more simpler, cleaner and correct. When archs want to move cpu_exec_init() and cpu_exec_exit() to realize/unlrealize, they can do so. Regards, Bharata.
