Re: [PATCH,RFC] random: collect cpu randomness

["Theodore Ts'o" <tytso@mit.edu>]

On Mon, Feb 03, 2014 at 10:50:42AM -0500, Jörn 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