On Tue, 14 Jul 2015 16:08:54 +0530 Bharata B Rao <bhar...@linux.vnet.ibm.com> wrote:
> On Thu, Jul 09, 2015 at 03:23:55PM +0200, Andreas Färber wrote: > > From: Bharata B Rao <bhar...@linux.vnet.ibm.com> > > > > 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. Newly added > > cpu_exec_exit() API handles the deallocation part and this routine is > > called from generic CPU instance_finalize. > > > > Note: This new CPU enumeration is for !CONFIG_USER_ONLY only. > > CONFIG_USER_ONLY continues to have the old enumeration logic. > > > > Signed-off-by: Bharata B Rao <bhar...@linux.vnet.ibm.com> > > Reviewed-by: Eduardo Habkost <ehabk...@redhat.com> > > Reviewed-by: Igor Mammedov <imamm...@redhat.com> > > Reviewed-by: David Gibson <da...@gibson.dropbear.id.au> > > Reviewed-by: Peter Crosthwaite <peter.crosthwa...@xilinx.com> > > Acked-by: Paolo Bonzini <pbonz...@redhat.com> > > Signed-off-by: Peter Crosthwaite <crosthwaite.pe...@gmail.com> > > [AF: max_cpus -> MAX_CPUMASK_BITS] > > Signed-off-by: Andreas Färber <afaer...@suse.de> > > --- > > exec.c | 58 > > ++++++++++++++++++++++++++++++++++++++++++++++++++----- > > include/qom/cpu.h | 1 + > > qom/cpu.c | 7 +++++++ > > 3 files changed, 61 insertions(+), 5 deletions(-) > > > > diff --git a/exec.c b/exec.c > > index ce5fadd..d817e5f 100644 > > --- a/exec.c > > +++ b/exec.c > > @@ -526,12 +526,57 @@ void tcg_cpu_address_space_init(CPUState *cpu, > > AddressSpace *as) > > } > > #endif > > > > +#ifndef CONFIG_USER_ONLY > > +static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS); > > + > > +static int cpu_get_free_index(Error **errp) > > +{ > > + int cpu = find_first_zero_bit(cpu_index_map, MAX_CPUMASK_BITS); > > + > > + if (cpu >= MAX_CPUMASK_BITS) { > > + error_setg(errp, "Trying to use more CPUs than max of %d", > > + MAX_CPUMASK_BITS); > > + return -1; > > + } > > If this routine and hence cpu_exec_init() (which is called from realize > routine) don't error out when max_cpus is reached, archs supporting CPU > hotplug using device_add will find it difficult to fail the realization of > CPU when hotplugging of more than max_cpus is attempted. > > An alternative is to explicitly check for the returned cpu_index > in realize call within each arch and fail if the cpu_index obtained > is greater than max_cpus. So for ppc, I could put such a check in > target-ppc/translate_init:ppc_cpu_realizefn(), but ppc_cpu_realizefn() > is a common routine for all targets under ppc and some targets like > ppc64abi32-linux-user don't have visibility to max_cpus which is > in vl.c. > > Any thoughts on the above problem ? we already have MachineClass.max_cpus which is max supported limit of machine type. Perhaps make max_cpus a property of MashineState > > Also, is it possible to revisit the problem that use of max_cpus instead > of MAX_CPUMASK_BITS caused to xlnx-ep108 ? > > Regards, > Bharata. > >