Messages in this thread | | | Date | Wed, 4 May 2022 07:36:53 +0200 | From | Dominik Brodowski <> | Subject | Re: [PATCH] random: mix hwgenerator randomness before sleeping |
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Hi Jason,
Am Wed, May 04, 2022 at 02:45:46AM +0200 schrieb Jason A. Donenfeld: > On Tue, May 3, 2022 at 9:56 PM Dominik Brodowski > <linux@dominikbrodowski.net> wrote: > > > > Am Tue, May 03, 2022 at 09:51:41PM +0200 schrieb Jason A. Donenfeld: > > > The add_hwgenerator_randomness() function is called in a loop from a > > > kthread by the hwgenerator core. It's supposed to sleep when there's > > > nothing to do, and wake up periodically for more entropy. Right now it > > > receives entropy, sleeps, and then mixes it in. This commit reverses the > > > order, so that it always mixes in entropy sooner and sleeps after. This > > > way the entropy is more fresh. > > > > ... however, the hwgenerator may take quite some time to accumulate entropy > > after wakeup. So now we might have a delay between a wakeup ("we need more > > entropy!") and that entropy becoming available. Beforehand, the thread only > > went to sleep when there is no current need for "fresh" entropy. > > Huh, interesting consideration. I didn't think about that. You wrote, > "hwgenerator may take quite some time to accumulate entropy" -- any > idea how long in the worst case? A second? A minute?
For TPM or virtio-rng, this shouldn't take long. But, for example, the Kconfig entry for HW_RANDOM_TIMERIOMEM states that
This driver provides kernel-side support for a generic Random Number Generator used by reading a 'dumb' iomem address that is to be read no faster than, for example, once a second; the default FPGA bitstream on the TS-7800 has such functionality.
Then, optee-rng.c contains a value "data_rate" in random bytes per second, and may sleep for a considerable time:
msleep((1000 * (max - read)) / pvt_data->data_rate);
Thanks, Dominik
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