[Xen-users] xen clocksource and TSC

["David A. Van Arnem" <dvanarnem@xxxxxxxxx>]

Hello all,

I'm working on manipulating the pv_soft_rdtsc() function in 
arch/x86/time.c.  In a paravirtualized 64-bit Fedora 20 domain, with 
tsc_mode="always_emulate" and "xen" as the current_clocksource, I find 
that if the domain is running and then the TSC value written to EAX/EDX 
is changed in pv_soft_rdtsc to be higher (faster) than normal, the 
passage of wallclock time in the domain appears faster (expected).  
However, if it is less than normal (slower), I would expect wallclock 
time to appear slower, but instead the domain freezes and eventually the 
watchdog timer is fired and it shuts down.  If I set the clocksource for 
Fedora to TSC, I see the passage of time slow down for a varying amount 
of time (sometimes seconds, sometimes minutes) but eventually again the 
domain hangs and the watchdog timer fires.  There are no messages in xl 
dmesg that indicate the domain is having problems (aside from the 
watchdog messages), so I'm having trouble determining what is causing 
this behavior.
I know that when the domain is created, there are time scales set to 
convert TSC ticks to real-time, and vice-versa, for timekeeping 
purposes.  I figured that if these scales remained the same, changing 
the TSC values would change wallclock time in the domain. What else 
might I need to consider, and/or, is this even possible on a running 
domain?  Does changing the TSC value in pv_soft_rdtsc affect other parts 
of the hypervisor (scheduling, etc) which might be affecting the 
domain?  I know this may not be easy to answer, but any direction or 
input would be very appreciated.
Thanks,
David


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