Re: [Qemu-devel] [patch] option -no-tsc for i386 with speedstep

[Kyle Hayes <kyle@silverbeach.net>]

On Monday 25 April 2005 04:15, Massimo Dal Zotto wrote:
> When qemu runs on an i386 cpu with speedstep enabled the clock of the
> guest os is not in sync with the clock on the host os because the
> vm_timer used for irq 0 generates interrupts at wrong rate when
> the host cpu frequency changes.
>
> The problem is that the vm_timer uses the rdtsc instruction and the
> value of ticks_per_sec, computed at start time, for calculating the
> expire time of vm_timers. While ticks_per_sec is constant the values
> returned by rdtsc are dependent on the current cpu clock, which is not
> constant if speedstep is used.

There is a discussion about this problem on either x86-secret.com or 
Sudhian.  I can't remember which :-(  In the latest series of P4 chips 
(both Xeon and desktop I think), the counter used for the rdtsc 
instruction stays at the same frequency while the rest of the CPU changes.  
This led to some "interesting" results with programs like CPU-Z.

Intel's response was that they now hold the counter at a stable frequency 
so that this works.  Unfortunately, there are years of processors which do 
not do that, so other workarounds must be done.

Best,
Kyle