linux-kernel - Re: [PATCH] random: mix all saved registers into entropy pool

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Date:	Mon, 19 May 2014 16:05:06 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jörn Engel <joern@...fs.org>
CC:	"Theodore Ts'o" <tytso@....edu>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] random: mix all saved registers into entropy pool

On 05/19/2014 03:39 PM, Jörn Engel wrote:
> On Mon, 19 May 2014 15:18:01 -0700, H. Peter Anvin wrote:
>>
>> I think this is likely to be particularly valuable during boot.  I can
>> see it becoming substantially less valuable after that point, but during
>> boot is when the most critical.
>>
>> What I do see as an issue is that the value is probably impossible to
>> quantify, and so I feel more than a bit queasy about giving it
>> /dev/random credit.  However, making sure the pool is well stirred
>> during boot is really way more important.
> 
> I would feel fairly confident giving this .25 bits of entropy per
> event.  With 40% unique hashes and assuming at most 1 bit of entropy
> for a unique hash, that is a fairly conservative underestimate.
> 

Sure, but that is for a specific workload.

> What worries me is that much of those 5s may be spent inside the idle
> loop and have rather predictable register content and we may run
> tickless.  I have no great solution for that case.
>
> We could change the ratelimiting to, say, 4 interrupts per jiffie.
> But that would still not help much in the worst case scenario above,
> so the benefits don't seem convincing.

Exactly.

	-hpa

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