Re: [patch 3/5] KVM: hypercall batching (v2)

[Marcelo Tosatti <marcelo@kvack.org>]

On Thu, Feb 21, 2008 at 08:30:04PM +0200, Avi Kivity wrote:
> >Perhaps you want to move that enforcement to the host.
> >
> >This allows batching of future hypercalls (if appropriate) to be easy.
> >
> >  
> 
> I'm still uneasy about it, though I have no rational reasons left now.
> 
> Oh, there is one: with a MMU_OP hypercall you can take the mmu spinlock 
> once per batch (dropping it once in a while to let another vcpu make 
> progress or to inject an interrupt).

emulator_write_phys() needs to do blocking work for each pte
(mmu_guess_page_from_pte), so that optimization would need quite some
work (separate mmu_guess_page_from_pte from kvm_mmu_pte_write).

And you can do this optimization even without MMU_OP, just check in
advance how many operations will take the mmu lock, do the non-blocking
part of them, and then manipulate the mmu lock protected shadow data.

Also, holding the spinlock for a longer period is not necessarily an
improvement (with the slots_lock it clearly is because there is no write
contention).


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