Re: [Qemu-devel] [PULL v3 05/22] cpu: Convert cpu_index into a bitmap

[Igor Mammedov]

On Wed, 15 Jul 2015 09:12:48 +0530
Bharata B Rao <address@hidden> wrote:

> On Tue, Jul 14, 2015 at 5:17 PM, Igor Mammedov <address@hidden>
> wrote:
> > On Tue, 14 Jul 2015 16:08:54 +0530
> > Bharata B Rao <address@hidden> wrote:
> >
> >> On Thu, Jul 09, 2015 at 03:23:55PM +0200, Andreas Färber wrote:
> >> > From: Bharata B Rao <address@hidden>
> >> >
> >> > 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 <address@hidden>
> >> > Reviewed-by: Eduardo Habkost <address@hidden>
> >> > Reviewed-by: Igor Mammedov <address@hidden>
> >> > Reviewed-by: David Gibson <address@hidden>
> >> > Reviewed-by: Peter Crosthwaite <address@hidden>
> >> > Acked-by: Paolo Bonzini <address@hidden>
> >> > Signed-off-by: Peter Crosthwaite <address@hidden>
> >> > [AF: max_cpus -> MAX_CPUMASK_BITS]
> >> > Signed-off-by: Andreas Färber <address@hidden>
> >> > ---
> >> >  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
> 
> MachineClass.max_cpus is the maximum number of CPUs supported for the
> machine. What we want to check here is against the max_cpus that the
> guest has booted with.
> 
> So are you suggesting that we move vl.c:max_cpus to
> MachineState.max_cpus and use that from all archs instead of using
> vl.c:max_cpus directly ?
yep, along the way move related  cpus,threads,sockets,cores from vl.c to
MachineState

> 
> Regards,
> Bharata.