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

[Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>]

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@intel.com>; dev@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@dpdk.org] On Behalf Of Zhihong Wang
>>>> Sent: Wednesday, November 18, 2015 3:27 AM
>>>> To: dev@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.

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.
>>> --