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

[Eric Blake]

On 03/05/2013 01:03 PM, mdroth wrote:

>> +        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.

Actually, there is both.  Here's what I have on my 2-cpu laptop, running
Fedora 18:

# find /sys/devices/system/cpu/ -name online
/sys/devices/system/cpu/cpu1/online
/sys/devices/system/cpu/online

Notice that there is NO /sys/devices/system/cpu/cpu0/online, because
this particular laptop's chipset requires that cpu0 ALWAYS be online.
The per-cpu online file exists only for cpus that can safely be
offlined; if it does not exist, then you must leave that cpu on.

> 
> Is that file also available on the distro you're testing with? Hopefully
> there's a single interfaces we can rely on.

Libvirt also relies on the per-cpu online files, and hasn't had any
complaints across the distros.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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