linux-kernel - Re: [PATCH] kvm mmu: reduce 50% memory usage

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Date:	Wed, 28 Apr 2010 15:05:03 -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 mmu: reduce 50% memory usage

On Wed, Apr 28, 2010 at 07:57:01PM +0800, Lai Jiangshan wrote:
> 
> I think users will enable tdp when their hardwares support ept or npt.
> This patch can reduce about 50% kvm mmu memory usage for they.
> 
> This simple patch use the fact that:
> 
> When sp->role.direct is set, sp->gfns does not contain any essential
> information, leaf sptes reachable from this sp are for a continuate
> guest physical memory range(a linear range).
> So sp->gfns[i](if it was set) equals to sp->gfn + i. (PT_PAGE_TABLE_LEVEL)
> Obviously, it is not essential information, we can calculate it when need.
> 
> It means we don't need sp->gfns when sp->role.direct=1,
> Thus we can save one page usage for every kvm_mmu_page.
> 
> Note:
> Access to sp->gfns must be wrapped by kvm_mmu_page_get_gfn()
> or kvm_mmu_page_set_gfn().
> It is only exposed in FNAME(sync_page).
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>

Applied, thanks.

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