Re: [Patch v5.1 03/03]: hwrng: khwrngd derating per device

[Ingo Tuchscherer]

Hi @all,

First, sorry for the late feedback, I was busy with other topics.

In the meanwhile I had a look at this hwrng entropy feature related to s390
and the zcrypt hardware random generator.
I have a concern about the initial call to the hwrng device while trying to
register the same.

drivers/char/hw_random/core.c
...
int hwrng_register(struct hwrng *rng) {
   ...
   bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
        if (bytes_read > 0)
                add_device_randomness(bytes, bytes_read);
  ...
}

At this point in time the zcrypt hw device is not registered completely in
the internal zcrypt device list (ap_device_list). The crypto card itself is
initialized and ready to receive and service requests, but the tasklet that
retrieve the card response is not able to find the device because it's not
yet in the list. Finally the response would not be received and the thread
is still waiting for completion, hence the probing/registering procedure
for all the other crypto devices is blocked. No further devices could be
scanned, initialized and registered.
Therefore I would not recommend to trigger a hw device request before the
device register process has completed.

Anyway, when the kernel thread (hwrng_fillfn) starts, it take care about
the initial call to the zcrypt rng device to be triggered. Therefore I
don't see any reason to manually call rng_get_data() during the
registration or did I missed something?

Mit freundlichen Grüßen / Kind regards

Ingo Tuchscherer

Software Development - Linux on System z
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From:	"Theodore Ts'o" <tytso@xxxxxxx>
To:	Torsten Duwe <duwe@xxxxxxx>,
Cc:	Ingo Tuchscherer/Germany/IBM@IBMDE, Hans-Georg
            Markgraf/Germany/IBM@IBMDE, geralds@xxxxxxxxxxxxxxxxxx,
            mschwid2@xxxxxxxxxxxxxxxxxx, heicars2@xxxxxxxxxxxxxxxxxx, Jörn
            Engel <joern@xxxxxxxxx>
Date:	16.06.2014 16:28
Subject:	Re: [Patch v5.1 03/03]: hwrng: khwrngd derating per device



On Mon, Jun 16, 2014 at 04:14:44PM +0200, Torsten Duwe wrote:
> Also, I added the patch below to our kernel. Who wants to take that
> one-liner upstream?

This will require sucking in the "master" branch of random.git.  So
either the s390 tree can merge in the master branch, or I can carry it
in the random.git tree.  I'm happy to do that unless their are any
objections.

(Note: if you want to base something on random.git, please use the
master branch.  The dev branch is sometimes rewound...  I've since
moved master up to the dev branch, which is my guarantee not to rewind
anything up to the master branch.)

> --- linux-3.12-SLE12/drivers/s390/crypto/zcrypt_api.c.orig
2014-06-16 14:23:27.726162185 +0200
> +++ linux-3.12-SLE12/drivers/s390/crypto/zcrypt_api.c
2014-06-16 14:46:31.869103924 +0200
> @@ -1374,6 +1374,7 @@ static int zcrypt_rng_data_read(struct h
>  static struct hwrng zcrypt_rng_dev = {
>  		 .name		 		 = "zcrypt",
>  		 .data_read		 = zcrypt_rng_data_read,
> +		 .quality		 = 990,
>  };
>
>  static int zcrypt_rng_device_add(void)

I assume the z-Series RNG is presumed to be high quality, yes?

  		      		       		      		       -
Ted

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