Re: In kernel PIC support: kernel patch

Gregory Haskins
2007-06-25T22:36:48Z

On Mon, 2007-06-25 at 18:25 -0400, Avi Kivity wrote:
> Gregory Haskins wrote:
> > On Mon, 2007-06-25 at 18:08 -0400, Avi Kivity wrote:
> >
> >> There's a slight issue that Windows with the ACPI HAL
> >> is dog slow when virtulalized, but maybe we can find another HAL that
> >> supports ACPI but doesn't use the APIC.
> >>
> >
> > Is this in reference to the frequent TPR updates, or is there something
> > else that is also slow with ACPI enabled?
> >
> >
>
> Just the good old tpr.

Ah, yes. I actually find the later versions of KVM to perform quite
well even with XP+ACPI. I think its all the work that went into the VMX
light-exits, mostly. Things seem to run even better with the in-kernel
APIC patch enabled, but admittedly I haven't run any benchmarks on it to
tell for sure. This is all "seat of the pants" ;). I am not exactly
sure why the in-kernel APIC would/could make a difference since TPRs are
light-exits in either case IIUC...

I am very interested to see what kind of gains we can get when TPR
shadowing is implemented properly (my attempt wasn't even fully
functional if you recall). I seem to recall reading somewhere that
TPR-shadow features can work even with 32 bit OSes (I guess MOV-TO-CR8
works in 32-bit mode?) but I don't know if XP would use it or the MMIO
path. Im guessing MMIO, but perhaps MS will patch it since they are
interested in VMM technology now. Does AMD support a similar notion of
TPR-shadow?

-Greg