RE: [Xen-devel] TPR write optimization (even improves 2003 sp2)

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> > I am probably showing a lack of understanding about how the APIC
works
> > here, but from a very brief look at what writing to the TPR register
> > does, all I can see is that it sets a value in vlapic->regs->data.
Could
> > I just map a page from the DomU into xen and assign a byte in there
as
> > the TPR register (one byte per CPU). I would need a new hypercall to
> > turn this on but that's no big deal. Or is there more to a TPR write
> > than just setting vlapic->regs->data?
> 
> Sometimes it triggers an interrupt to be delivered. So you'd need a
TPR
> threshold too to cause a VMEXIT sometimes. Or perhaps map the whole
APIC
> page and work it out yourself (possibly the Citrix drivers do this,
but
> I'm not really familiar with them).

I can't see anything that would trigger an interrupt beyond
vlapic_write(). Is it just the act of invoking a VMEXIT that triggers
the interrupt delivery?

>From memory, there is a register somewhere that says what the highest
interrupt level that is currently blocked is... I'll look at that. This
doesn't actually sound that hard...

James

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