Re: windows acpi time drift

[Dor Laor <dor.laor@gmail.com>]

On Wed, 2008-03-19 at 17:39 +0200, Avi Kivity wrote:
> Anthony Liguori wrote:
> > Avi Kivity wrote:
> >>   The fourth is probably impossible from userspace (and very 
> >> difficult in the kernel).
> >
> > What makes it impossible to do in userspace?  If you managed a 
> > tsc_offset in userspace, you would of course need to adjust that 
> > tsc_offset within the kernel for the particular PCPU that you were on.
> >
> 
> In the kernel you can to tricks like local_irq_disable(); rdtsc(); 
> ktime_get(); local_irq_enable() to get a sense where the tsc is.

but you can also do it before the vcpu goes to userspace after vmexit.

> 
> Take a look at kvm_inject_pit_timer_irqs() and 
> kvm_pit_timer_intr_post().  An attempt to have a accurate userspace pit 
> needs to take into account what those functions do.  I believe it's 
> doable, but will require careful design of the interface (which should 
> be usable for rtc and hpet as well).
> 

Actually I'm coming to think we don't need a irq queue in the kernel.
We just need to count the pending timer interrupts in userspace and
change the qemu_set_irq interface to return a status when the irq was
really injected by pic/apic (like kvm_pit_timer_intr_post).

This way qemu timer devices will not inject another irq until the
previous irq got ack by the kernel (or even userspace pic/acpi).



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