[dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove unnecessary hugepage zero-filling

[Xie, Huawei]

On 11/19/2015 5:15 PM, Gonzalez Monroy, Sergio wrote:
> On 18/11/2015 12:07, Xie, Huawei wrote:
>> On 11/18/2015 6:45 PM, Wang, Zhihong wrote:
>>>> -----Original Message-----
>>>> From: Mcnamara, John
>>>> Sent: Wednesday, November 18, 2015 6:40 PM
>>>> To: Wang, Zhihong <zhihong.wang at intel.com>; dev at dpdk.org
>>>> Subject: RE: [dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove
>>>> unnecessary
>>>> hugepage zero-filling
>>>>
>>>>
>>>>
>>>>> -----Original Message-----
>>>>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Zhihong Wang
>>>>> Sent: Wednesday, November 18, 2015 3:27 AM
>>>>> To: dev at dpdk.org
>>>>> Subject: [dpdk-dev] [RFC PATCH 2/2] lib/librte_eal: Remove
>>>>> unnecessary
>>>>> hugepage zero-filling
>>>>>
>>>>> The kernel fills new allocated (huge) pages with zeros.
>>>>> DPDK just has to touch the pages to trigger the allocation.
>> I think we shouldn't reply on the assumption that kernel has zeroed the
>> memory. Kernel zeroes the memory mostly to avoid information leakage.It
>> could also achieve this by setting each bit to 1.
>> What we indeed need to check is later DPDK initialization code doesn't
>> assume the memory has been zeroed. Otherwise zero only that part of the
>> memory. Does this makes sense?
>
> Why cannot we rely on the kernel zeroing the memory ?
> If that behavior were to change, then we can zero out the memory
> ourselves.
It is undocumented kernel behavior. My opinion is if not a big burden,
zero out the needed memory ourselves, otherwise resort to this kernel
behavior.

>
> Bruce pointed out to me that the semantics have changed a bit since we
> introduced
> rte_memzone_free.
> Before that, all memzone reserve were zero out by default.
> Is there code relying on that? I'm not sure, but it still is good
> practice to do it.
>
> I submitted an RFC regarding this:
> http://dpdk.org/ml/archives/dev/2015-November/028416.html
>
> The idea would be to keep the available memory we are managing zeroed
> at all times.
>
> Sergio
>>>>> ...
>>>>>           if (orig) {
>>>>>               hugepg_tbl[i].orig_va = virtaddr;
>>>>> -            memset(virtaddr, 0, hugepage_sz);
>>>>> +            memset(virtaddr, 0, 8);
>>>>>           }
>>>> Probably worth adding a one or two line comment here to avoid someone
>>>> thinking that it is a bug at some later stage. The text in the
>>>> commit message
>>>> above is suitable.
>>>>
>>> Good suggestion! Will add it :)
>>>
>>>> John.
>>>> -- 
>
>