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

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Date:	Fri, 07 May 2010 16:25:36 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Marcelo Tosatti <mtosatti@...hat.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

Marcelo Tosatti wrote:
> On Thu, May 06, 2010 at 03:03:48PM +0800, Lai Jiangshan wrote:
>> Marcelo Tosatti wrote:
>>> 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.
>>>
>> Could you tell me how you test it? It will be better if I follow
>> your test steps.
> 
> mount -t hugetlbfs none /mnt/
> echo xyz > /proc/sys/vm/nr_hugepages
> qemu-kvm parameters -mem-path /mnt/
> 
>> I also hit the kvm_mmu_page_set_gfn BUG_ON, It is because
>> FNAME(fetch)() set sp->gfn wrong. The patch:
>> [PATCH] kvm: calculate correct gfn for small host pages which emulates large guest pages
>> fix it.
>>
>> I can not hit kvm_mmu_page_set_gfn BUG_ON after this patch also
>> applied.
>>
>> So could you tell me your test steps:
>> The host: ept/npt enabled? 64bit? testing codes in host?
> 
> Intel EPT enabled.
> 
>> The guest: OS? PAE? 32bit? 64bit? testing codes in guest?
> 
> FC12 guest.
> 


I forgot to check if the calculation of base gfn is correct
in __direct_map().

Subject: [PATCH] kvm, tdp: calculate correct base gfn for non-DIR level

the base gfn calculation is incorrect in __direct_map(),
it does not calculate correctly when level=3 or 4.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
Reported-by: Marcelo Tosatti <mtosatti@...hat.com>
---
diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index a5c6719..6986a6f 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -1955,7 +1956,10 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
 		}
 
 		if (*iterator.sptep == shadow_trap_nonpresent_pte) {
-			pseudo_gfn = (iterator.addr & PT64_DIR_BASE_ADDR_MASK) >> PAGE_SHIFT;
+			u64 base_addr = iterator.addr;
+
+			base_addr &= PT64_LVL_ADDR_MASK(iterator.level);
+			pseudo_gfn = base_addr >> PAGE_SHIFT;
 			sp = kvm_mmu_get_page(vcpu, pseudo_gfn, iterator.addr,
 					      iterator.level - 1,
 					      1, ACC_ALL, iterator.sptep);



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