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

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Message-ID: <20100430154427.GA32340@amt.cnet>
Date:	Fri, 30 Apr 2010 12:44:27 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH] kvm mmu: reduce 50% memory usage

On Thu, Apr 29, 2010 at 09:43:40PM +0300, Avi Kivity wrote:
> On 04/29/2010 09:09 PM, Marcelo Tosatti wrote:
> >
> >You missed quadrant on 4mb large page emulation with shadow (see updated
> >patch below).
> 
> Good catch.
> 
> >Also for some reason i can't understand the assumption
> >does not hold for large sptes with TDP, so reverted for now.
> 
> It's unrelated to TDP, same issue with shadow.  I think the
> calculation is correct.  For example the 4th spte for a level=2 page
> will yield gfn=4*512.

Under testing i see sp at level 2, with sp->gfn == 4096, mmu_set_spte
setting index 8 to gfn 4096 (whereas kvm_mmu_page_get_gfn returns 4096 +
8*512).

Lai, can you please take a look at it? You should see the
kvm_mmu_page_set_gfn BUG_ON by using -mem-path on hugetlbfs.

> >@@ -393,6 +393,27 @@ static void mmu_free_rmap_desc(struct kvm_rmap_desc *rd)
> >  	kfree(rd);
> >  }
> >
> >+static gfn_t kvm_mmu_page_get_gfn(struct kvm_mmu_page *sp, int index)
> >+{
> >+	gfn_t gfn;
> >+
> >+	if (!sp->role.direct)
> >+		return sp->gfns[index];
> >+
> >+	gfn = sp->gfn + index * (1<<  (sp->role.level - 1) * PT64_LEVEL_BITS);
> >+	gfn += sp->role.quadrant<<  PT64_LEVEL_BITS;
> 
> PT64_LEVEL_BITS * level
> 
> >+
> >+	return gfn;
> >+}
> >+
> >
> 
> 
> -- 
> I have a truly marvellous patch that fixes the bug which this
> signature is too narrow to contain.
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