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

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

On Thu, Jul 09, 2015 at 03:23:55PM +0200, Andreas Färber wrote:
> From: Bharata B Rao <bharata@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 <bharata@linux.vnet.ibm.com>
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
> Reviewed-by: Igor Mammedov <imammedo@redhat.com>
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
> [AF: max_cpus -> MAX_CPUMASK_BITS]
> Signed-off-by: Andreas Färber <afaerber@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 ?

Also, is it possible to revisit the problem that use of max_cpus instead
of MAX_CPUMASK_BITS caused to xlnx-ep108 ?

Regards,
Bharata.