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From: Avi Kivity on 3 Aug 2010 03:00
On 08/03/2010 05:30 AM, Lai Jiangshan wrote:
> This patch is just a big cleanup. it reduces 220 lines of code.
>
> It introduces sibling_pte array for tracking identical sptes, so the
> identical sptes can be linked as a single linked list by their
> corresponding sibling_pte. A reverse map or a parent_pte points at
> the head of this single linked list. So we can do cleanup for
> reverse map and parent_pte VERY LARGELY.
>
> BAD:
> If most rmap have only one entry or most sp have only one parent,
> this patch may use more memory than before.

That is the case with NPT and EPT. Each page has exactly one spte
(except a few vga pages), and each sp has exactly one parent_pte (except
the root pages).

> GOOD:
> 1) Reduce a lot of code, The functions which are in hot path becomes
> very very simple and terrifically fast.
> 2) rmap_next(): O(N) -> O(1). traveling a ramp: O(N*N) -> O(N)

The existing rmap_next() is not O(N), it's O(RMAP_EXT), which is 4. The
data structure was chosen over a simple linked list to avoid extra cache
misses.

> 3) Remove the ugly interlayer: struct kvm_rmap_desc, struct kvm_pte_chain

kvm_rmap_desc and kvm_pte_chain are indeed ugly, but they do save a lot
of memory and cache misses.

> 4) We don't need to allocate any thing when we change the mappings.
> So we can avoid allocation when we have held kvm mmu spin lock.
> (this feature is very helpful in future).
> 5) better readability.

I agree the new code is more readable. Unfortunately it uses more
memory and is likely to be slower. You add a cache miss for every spte,
while kvm_rmap_desc amortizes the cache miss among 4 sptes, and special
cases 1 spte to have no cache misses (or extra memory requirements).

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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From: Lai Jiangshan on 3 Aug 2010 05:00
On 08/03/2010 02:51 PM, Avi Kivity wrote:
> On 08/03/2010 05:30 AM, Lai Jiangshan wrote:
>> This patch is just a big cleanup. it reduces 220 lines of code.
>>
>> It introduces sibling_pte array for tracking identical sptes, so the
>> identical sptes can be linked as a single linked list by their
>> corresponding sibling_pte. A reverse map or a parent_pte points at
>> the head of this single linked list. So we can do cleanup for
>> reverse map and parent_pte VERY LARGELY.
>>
>> BAD:
>> If most rmap have only one entry or most sp have only one parent,
>> this patch may use more memory than before.
>
> That is the case with NPT and EPT. Each page has exactly one spte
> (except a few vga pages), and each sp has exactly one parent_pte (except
> the root pages).
>
>> GOOD:
>> 1) Reduce a lot of code, The functions which are in hot path becomes
>> very very simple and terrifically fast.
>> 2) rmap_next(): O(N) -> O(1). traveling a ramp: O(N*N) -> O(N)
>
> The existing rmap_next() is not O(N), it's O(RMAP_EXT), which is 4. The
> data structure was chosen over a simple linked list to avoid extra cache
> misses.
>
>> 3) Remove the ugly interlayer: struct kvm_rmap_desc, struct
>> kvm_pte_chain
>
> kvm_rmap_desc and kvm_pte_chain are indeed ugly, but they do save a lot
> of memory and cache misses.
>
>> 4) We don't need to allocate any thing when we change the mappings.
>> So we can avoid allocation when we have held kvm mmu spin lock.
>> (this feature is very helpful in future).
>> 5) better readability.
>
> I agree the new code is more readable. Unfortunately it uses more
> memory and is likely to be slower. You add a cache miss for every spte,
> while kvm_rmap_desc amortizes the cache miss among 4 sptes, and special
> cases 1 spte to have no cache misses (or extra memory requirements).
>

You are right, please omit this patch

thanks, lai.
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