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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 08/01/2009 00:40, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

> I just quickly implemented this as a 'proof of concept' by adding a
> XENMAPSPACE_vlapic_regs function to XENMEM_add_to_physmap, and it works,
> but only after I removed regs_page from the vlapic struct and used
> alloc_xenheap_page instead of alloc_domheap_page and then
> share_xen_page_with_guest. Can I do it with alloc_domheap_page? It
> didn't 'just work' when I did it that way... (and I had to comment out
> the use of regs_page in the vmx code to get it to compile, so as it
> stands it probably wouldn't work on Intel).

Using alloc_xenheap_page would be okay.

> Anyway, in my xp guest when I KeRaiseIrql and then read the mapped
> vlapic->regs from xp I see that the TPR register is changing, so things
> are looking good.
> 
> Next will be to patch windows to:
> . On read, just read from the mapped space instead of the mmio space for
> at least the TPR register, and maybe others if there are potential
> performance gains
> . On write, write the value to the mapped space, then check if an
> interrupt could be pending (as per Keirs algorithm in a previous email),
> and if so, write to the mmio space to force a VMEXIT (or use some other
> way of invoking a VMEXIT if it's faster...). By doing the mapped write
> first before checking for pending interrupts I should avoid any races...

Yes, sounds okay.

 K.



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