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Message-ID: <50A5C518.7030002@linux.vnet.ibm.com>
Date: Fri, 16 Nov 2012 12:46:16 +0800
From: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To: Marcelo Tosatti <mtosatti@...hat.com>
CC: Avi Kivity <avi@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: MMU: lazily drop large spte
On 11/16/2012 11:56 AM, Marcelo Tosatti wrote:
> On Fri, Nov 16, 2012 at 11:39:12AM +0800, Xiao Guangrong wrote:
>> On 11/16/2012 11:02 AM, Marcelo Tosatti wrote:
>>> On Thu, Nov 15, 2012 at 07:17:15AM +0800, Xiao Guangrong wrote:
>>>> On 11/14/2012 10:37 PM, Marcelo Tosatti wrote:
>>>>> On Tue, Nov 13, 2012 at 04:26:16PM +0800, Xiao Guangrong wrote:
>>>>>> Hi Marcelo,
>>>>>>
>>>>>> On 11/13/2012 07:10 AM, Marcelo Tosatti wrote:
>>>>>>> On Mon, Nov 05, 2012 at 05:59:26PM +0800, Xiao Guangrong wrote:
>>>>>>>> Do not drop large spte until it can be insteaded by small pages so that
>>>>>>>> the guest can happliy read memory through it
>>>>>>>>
>>>>>>>> The idea is from Avi:
>>>>>>>> | As I mentioned before, write-protecting a large spte is a good idea,
>>>>>>>> | since it moves some work from protect-time to fault-time, so it reduces
>>>>>>>> | jitter. This removes the need for the return value.
>>>>>>>>
>>>>>>>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>>>>>>>> ---
>>>>>>>> arch/x86/kvm/mmu.c | 34 +++++++++-------------------------
>>>>>>>> 1 files changed, 9 insertions(+), 25 deletions(-)
>>>>>>>
>>>>>>> Its likely that other 4k pages are mapped read-write in the 2mb range
>>>>>>> covered by a read-only 2mb map. Therefore its not entirely useful to
>>>>>>> map read-only.
>>>>>>>
>>>>>>
>>>>>> It needs a page fault to install a pte even if it is the read access.
>>>>>> After the change, the page fault can be avoided.
>>>>>>
>>>>>>> Can you measure an improvement with this change?
>>>>>>
>>>>>> I have a test case to measure the read time which has been attached.
>>>>>> It maps 4k pages at first (dirt-loggged), then switch to large sptes
>>>>>> (stop dirt-logging), at the last, measure the read access time after write
>>>>>> protect sptes.
>>>>>>
>>>>>> Before: 23314111 ns After: 11404197 ns
>>>>>
>>>>> Ok, i'm concerned about cases similar to e49146dce8c3dc6f44 (with shadow),
>>>>> that is:
>>>>>
>>>>> - large page must be destroyed when write protecting due to
>>>>> shadowed page.
>>>>> - with shadow, it does not make sense to write protect
>>>>> large sptes as mentioned earlier.
>>>>>
>>>>
>>>> This case is removed now, the code when e49146dce8c3dc6f44 was applied is:
>>>> |
>>>> | pt = sp->spt;
>>>> | for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
>>>> | /* avoid RMW */
>>>> | if (is_writable_pte(pt[i]))
>>>> | update_spte(&pt[i], pt[i] & ~PT_WRITABLE_MASK);
>>>> | }
>>>>
>>>> The real problem in this code is it would write-protect the spte even if
>>>> it is not a last spte that caused the middle-level shadow page table was
>>>> write-protected. So e49146dce8c3dc6f44 added this code:
>>>> | if (sp->role.level != PT_PAGE_TABLE_LEVEL)
>>>> | continue;
>>>> |
>>>> was good to fix this problem.
>>>>
>>>> Now, the current code is:
>>>> | for (i = 0; i < PT64_ENT_PER_PAGE; ++i) {
>>>> | if (!is_shadow_present_pte(pt[i]) ||
>>>> | !is_last_spte(pt[i], sp->role.level))
>>>> | continue;
>>>> |
>>>> | spte_write_protect(kvm, &pt[i], &flush, false);
>>>> | }
>>>> It only write-protect the last spte. So, it allows large spte existent.
>>>> (the large spte can be broken by drop_large_spte() on the page-fault path.)
>>>>
>>>>> So i wonder why is this part from your patch
>>>>>
>>>>> - if (level > PT_PAGE_TABLE_LEVEL &&
>>>>> - has_wrprotected_page(vcpu->kvm, gfn, level)) {
>>>>> - ret = 1;
>>>>> - drop_spte(vcpu->kvm, sptep);
>>>>> - goto done;
>>>>> - }
>>>>>
>>>>> necessary (assuming EPT is in use).
>>>>
>>>> This is safe, we change these code to:
>>>>
>>>> - if (mmu_need_write_protect(vcpu, gfn, can_unsync)) {
>>>> + if ((level > PT_PAGE_TABLE_LEVEL &&
>>>> + has_wrprotected_page(vcpu->kvm, gfn, level)) ||
>>>> + mmu_need_write_protect(vcpu, gfn, can_unsync)) {
>>>> pgprintk("%s: found shadow page for %llx, marking ro\n",
>>>> __func__, gfn);
>>>> ret = 1;
>>>>
>>>> The spte become read-only which can ensure the shadow gfn can not be changed.
>>>>
>>>> Btw, the origin code allows to create readonly spte under this case if !(pte_access & WRITEABBLE)
>>>
>>> Regarding shadow: it should be fine as long as fault path always deletes
>>> large mappings, when shadowed pages are present in the region.
>>
>> For hard mmu is also safe, in this patch i added these code:
>>
>> @@ -2635,6 +2617,8 @@ static int __direct_map(struct kvm_vcpu *vcpu, gpa_t v, int write,
>> break;
>> }
>>
>> + drop_large_spte(vcpu, iterator.sptep);
>> +
>>
>> It can delete large mappings like soft mmu does.
>>
>> Anything i missed?
>>
>>>
>>> Ah, unshadowing from reexecute_instruction does not handle
>>> large pages. I suppose that is what "simplification" refers
>>> to.
>>
>> reexecute_instruction did not directly handle last spte, it just
>> removes all shadow pages, then let cpu retry the instruction, the
>> page can become writable when encounter #PF again, large spte is fine
>> under this case.
>
> While searching for a given "gpa", you don't find large gfn which is
> mapping it, right? (that is, searching for gfn 4 fails to find large
> read-only "gfn 0"). Unshadowing gfn 4 will keep large read-only mapping
> present.
>
> 1. large read-write spte to gfn 0
> 2. shadow gfn 4
> 3. write-protect large spte pointing to gfn 0
> 4. write to gfn 4
> 5. instruction emulation fails
> 5. unshadow gfn 4
> 6. refault, do not drop large spte because no pages shadowed
Hmm, it is not true. :)
The large spte can become writable since 'no pages adhadoes' (that means
has_wrprotected_page() can return 0 for this case). No?
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