Re: [Qemu-devel] [PATCH 2/3] qga: implement qmp_guest_get_vcpus() for Linux with sysfs

[Laszlo Ersek]

On 03/05/13 21:03, mdroth wrote:> On Mon, Mar 04, 2013 at 11:19:56PM +0100, 
Laszlo Ersek wrote:

>> +#if defined(__linux__)
>
> There's a section in commands-posix.c set aside under "linux-specific
> implementations" for these, and another underneath for stubs so we can
> avoid having too many ifdef's.

I'll check it, thanks.

>> +        buf = g_strdup_printf("/sys/devices/system/cpu/cpu%ld/online",
>> +                              current);
>> +        f = fopen(buf, "r");
>> +        if (f == NULL) {
>> +            error_setg_errno(&local_err, errno, "fopen(\"%s\", \"r\")", 
>> buf);
>> +        } else {
>> +            unsigned online;
>> +
>> +            if (fscanf(f, "%u", &online) != 1) {
>
> On Fedora 18 and Ubuntu 12.04 at least there doesn't seem to be per-cpu
> values for online/offline/etc, but instead just a 'global' entry at
> /sys/devices/system/cpu/{online,offline} that provides a range. This is
> what's currently described in
> linux/Documentation/ABI/testing/sysfs-devices-system-cpu as well.
>
> Is that file also available on the distro you're testing with? Hopefully
> there's a single interfaces we can rely on.

On RHEL-6, both "/sys/devices/system/cpu/cpu*/online" and
"/sys/devices/system/cpu/online " are available.

On RHEL-5, only "/sys/devices/system/cpu/cpu*/online" is available.

"/sys/devices/system/cpu/cpu*/online" is documented in
"Documentation/cpu-hotplug.txt", in all of RHEL-5, RHEL-6, and upstream
Linux.

            #pwd
            #/sys/devices/system/cpu
            #ls -l
            total 0
            drwxr-xr-x  10 root root 0 Sep 19 07:44 .
            drwxr-xr-x  13 root root 0 Sep 19 07:45 ..
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu0
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu1
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu2
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu3
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu4
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu5
            drwxr-xr-x   3 root root 0 Sep 19 07:44 cpu6
            drwxr-xr-x   3 root root 0 Sep 19 07:48 cpu7

    Under each directory you would find an "online" file which is the control
    file to logically online/offline a processor.

    Q: Does hot-add/hot-remove refer to physical add/remove of cpus?
    A: The usage of hot-add/remove may not be very consistently used in the 
code.
    CONFIG_HOTPLUG_CPU enables logical online/offline capability in the kernel.
    To support physical addition/removal, one would need some BIOS hooks and
    the platform should have something like an attention button in PCI hotplug.
    CONFIG_ACPI_HOTPLUG_CPU enables ACPI support for physical add/remove of 
CPUs.

The kernels you mention may not have CONFIG_HOTPLUG_CPU enabled
(consequently they would probably not support the functionality either).

... I just checked "/boot/config-3.8.2-201.fc18.x86_64" in
"kernel-3.8.2-201.fc18.x86_64.rpm" and all required config options seem
to be set. I checked on a much older F18 guest as well
(3.6.10-4.fc18.x86_64), and the per-cpu "online" files are there. (Not
for cpu0, but I'll address that in response to Eric's review.) I'm not
sure why you don't get them under F18.

Thanks
Laszlo