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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 07/01/2009 09:33, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

>> Hmmm, you could be right. I suppose xentrace can confirm one way or
> the
>> other once you have it set up...
>> 
> 
> 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).

 -- Keir



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