linux-kernel - Re: [PATCH] kvm: rework remove-write-access for a slot

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Date:	Fri, 4 Jun 2010 12:18:25 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
	kvm@...r.kernel.org
Subject: Re: [PATCH] kvm: rework remove-write-access for a slot

On Fri, Jun 04, 2010 at 04:14:08PM +0800, Lai Jiangshan wrote:
> Avi Kivity wrote:
> > On 06/02/2010 11:53 AM, Lai Jiangshan wrote:
> >> Current code uses slot_bitmap to find ptes who map a page
> >> from the memory slot, it is not precise: some ptes in the shadow page
> >> are not map any page from the memory slot.
> >>
> >> This patch uses rmap to find the ptes precisely, and remove
> >> the unused slot_bitmap.

Note that the current code is precise: memslot_id does unalias_gfn.

> > Patch looks good; a couple of comments:
> > 
> > - We might see a slowdown with !tdp, since we no longer have locality. 
> > Each page will map to an spte in a different page.  However, it's still
> > worth it in my opinion.
> 
> Yes, this patch hurts the cache since we no longer have locality.
> And if most pages of the slot are not mapped(rmap_next(kvm, rmapp, NULL)==NULL),
> this patch will worse than old method I think.

Can you get some numbers before/after patch, with/without lots of shadow
pages instantiated? Better with large amount of memory for the guest.

Because shrinking kvm_mmu_page is good.

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