[Xen-devel] System time monotonicity

[John Levon <levon@xxxxxxxxxxxxxxxxx>]

It seems that VCPU system time isn't monotonic (using 3.0.4). It seems
it might be correlated to when a VCPU is switched across real CPUs but I
haven't conclusively proved that. But e.g.:

    {
        old = {
            time = {
                version = 0x4ec
                pad0 = 0xe8e0
                tsc_timestamp = 0x22cc8398b7194
                system_time = 0xe8e0345d8805
                tsc_to_system_mul = 0xd62c0083
                tsc_shift = '\377'
                pad1 = [ '\002', '\027', '\365' ]
            }
            result = 0xe8e0484568fa
            tsc = 0x22cc86921ab00
            cpu = 0
        }
        new = {
            time = {
                version = 0x4ee
                pad0 = 0
                tsc_timestamp = 0x22cc7db96cd29
                system_time = 0xe8e00d1031f3
                tsc_to_system_mul = 0xd62ae844
                tsc_shift = '\377'
                pad1 = [ '\357', '\002', '\365' ]
            }
            result = 0xe8e048456012
            tsc = 0x22cc869225443
            cpu = 0
        }
        delta = 0xfffffffffffff718
    }

>From what I can work out, time is supposed to be monotonic but I admit I
can't really understand the time code yet at least. I couldn't find any
documentation on what to expect from system time. Any suggestions?

This seems to happen across all the hardware we've tried but this
particular case is a Sun V20Z with two CPUs:

  x86 (AuthenticAMD family 15 model 5 step 10 clock 2392 MHz)
        AMD Opteron(tm) Processor 250

cheers,
john

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