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Re: [Qemu-devel] [PULL v3 05/22] cpu: Convert cpu_index into a bitmap
From: |
Bharata B Rao |
Subject: |
Re: [Qemu-devel] [PULL v3 05/22] cpu: Convert cpu_index into a bitmap |
Date: |
Tue, 14 Jul 2015 16:08:54 +0530 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
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 ?
Also, is it possible to revisit the problem that use of max_cpus instead
of MAX_CPUMASK_BITS caused to xlnx-ep108 ?
Regards,
Bharata.
- Re: [Qemu-devel] [PULL v3 05/22] cpu: Convert cpu_index into a bitmap,
Bharata B Rao <=