Re: RFC: shadow page table reclaim

[Avi Kivity <avi@redhat.com>]

On 09/02/2009 05:24 AM, Max Laier wrote:
> Okay ... I have/something/, but I'm certainly not there yet as I have to
> admit that I don't understand your idea completely.  In addition it seems that
> EPT doesn't have an accessed bit :-\  Any idea for this?
>    

Use the rwx bits as an approximation.  If the pages are needed they'll 
be faulted back in, which is a lot cheaper than reconstructing them.

But why do you see reclaim with ept?  The pages ought to be constructed 
once and then left alone, unless there is severe memory pressure.

> Regardless, testing the attached with EPT, it turns out that not zapping
> shadow pages with root_count != 0 already makes much difference.  After all we
> don't really zap these pages anyways, but just mark them invalid after zapping
> the children.  So this could be a first improvement.
>
> In any case, I clearly don't have the right idea here, yet.  Plus I don't
> really have time to look into this further right now.  And my hack is "good
> enough"[tm] for my testing ... so if anyone more knowledgeable would like to
> continue - much appreciated.  Maybe some of this can at least serve as food
> for thoughts.  Sorry.
>    

Sure.

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a3f637f..089ad0e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -394,6 +394,7 @@ struct kvm_arch{
>   	 * Hash table of struct kvm_mmu_page.
>   	 */
>   	struct list_head active_mmu_pages;
> +	struct kvm_mmu_page *scan_hand;
>   	struct list_head assigned_dev_head;
>   	struct iommu_domain *iommu_domain;
>   	int iommu_flags;
>    

Why is a scan hand needed?  I though you could just clear the accessed 
bits and requeue the page.

If you drop a page, all the accessed bits in the ptes are lost with it, 
so you need to transfer them to the pointed-to pages before you dropped 
it.  Other than that, this seems pretty complete.

-- 
error compiling committee.c: too many arguments to function