On 06/19/2013 07:58 PM, Paolo Bonzini wrote:
> Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
>> Document write_flooding_count to Documentation/virtual/kvm/mmu.txt
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
>> ---
>> Documentation/virtual/kvm/mmu.txt | 8 ++++++++
>> arch/x86/include/asm/kvm_host.h | 5 +++++
>> 2 files changed, 13 insertions(+)
>>
>> diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
>> index ce6df51..5a6b2e2 100644
>> --- a/Documentation/virtual/kvm/mmu.txt
>> +++ b/Documentation/virtual/kvm/mmu.txt
>> @@ -214,6 +214,14 @@ Shadow pages contain the following information:
>> It is only used on 32bit host which helps us to detect whether updating the
>> 64bit spte is complete so that we can avoid reading the truncated value out
>> of mmu-lock.
>> + write_flooding_count:
>> + In order to sync the page table between guest and host, the page sometimes
>> + needs to be write-protected (see "Synchronized and unsynchronized pages"
>> + below), any write to the page can cause write emulation. If the emulation
>> + on the page is too frequent we'd better unmap the page to avoid the
>> + future emulation. write_flooding_count aims at this optimization which is
>> + increased when the page needs to be write emulated and cleared when the
>> + page is actually used.
>
> A guest may write to a page table many times, causing a lot of
> emulations if the page needs to be write-protected (see "Synchronized
> and unsynchronized pages" below). Leaf pages can be unsynchronized
> so that they do not trigger frequent emulation, but this is not
> possible for non-leafs. This field counts the number of emulations
> since the last time the page table was actually used; if emulation
> is triggered too frequently on this page, KVM will unmap the page
> to avoid emulation in the future.
very nice.
>
>> Reverse map
>> ===========
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 1dac2c1..5eb5382 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -234,6 +234,11 @@ struct kvm_mmu_page {
>> int clear_spte_count;
>> #endif
>>
>> + /*
>> + * Increased when the page needs to be write emulated, cleared
>> + * when the page is actually used as page table to avoid frequent
>> + * emulation on the page.
>> + */
>
> /* Number of writes since the last time traversal visited this page. */
>
> wdyt, better or worse?
I think it is better. :)
Thanks for your refining.
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