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Date: Mon, 3 Feb 2014 11:37:29 -0500
From: "Theodore Ts'o" <tytso@mit.edu>
To: =?iso-8859-1?Q?J=F6rn?= Engel <joern@logfs.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel Developers List <linux-kernel@vger.kernel.org>,
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        blogic@openwrt.org, andrewmcgr@gmail.com, smueller@chronox.de,
        geert@linux-m68k.org, tg@mirbsd.de
Subject: Re: [PATCH,RFC] random: collect cpu randomness
Message-ID: <20140203163729.GA13634@thunk.org>
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	=?iso-8859-1?Q?J=F6rn?= Engel <joern@logfs.org>,
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On Mon, Feb 03, 2014 at 10:50:42AM -0500, J?rn Engel wrote:
> If the measurement event is an interrupt and the CPU has a
> cycle-counter, you are set.  On interesting systems lacking a
> cycle-counter, we still have a high-resolution counter or sorts that
> is the CPU itself.
> 
> Instruction pointer and stack pointer for both kernel and userland are
> one way to read out the "counter".  Main problem here are tight loops
> where your "counter" is not high-resolution at all.  But something
> within the CPU is constantly changing.  And that something tends to be
> contained in the registers.
> 
> How about taking the saved registers from the interrupted CPU, xor'ing
> them all and calling the result random_get_entropy() on systems
> lacking a good cycles-counter?

So we could take the struct pt_regs which we get from get_irq_regs(),
XOR them together and use them to feed into input[2] amd input[3] in
add_interrupt_randomness().  Or some other way of distributing the
values of all of the irq registers into the __u32 input[4] array.

That would probably be a good and useful thing to do.  Was that
basically what you were suggesting?

						- Ted
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