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

[Fabrice Bellard]

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.
>
> The problem can be cleary observed by running "xclock -update 1" in the
> guest os and observing how the clock speed varies with the cpu freqency.
>
> The following patch fixes the problem by adding a new option -no-tsc for
> the i386 architecture which can be used to disable rdtsc when running on
> a cpu with speedstep enabled.
>
> The patch works for me but I don't know if this is the best way of fixing
> this bug. If anyone has a better suggestion it is welcome.

This is not the best way to fix the bug. I see 4 solutions:

- Compute ticks_per_sec every n seconds and update the timers accordingly.

- Use a virtual cycle counter and update ticks_per_sec dynamically. This 
solution has the advantage of not needing any precise host timer (= very 
portable), but it will slow down QEMU a bit.

- Use a specific driver to get the time efficiently (= without system 
call) or to know when the host frequency changes

- Disable speedstep or use a CPU for which the rdtsc period does not 
vary even if the CPU frequency changes (the P4 has this feature if I 
remember correctly and all well designed CPUs should have such a feature !).

Fabrice.