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Date:   Tue, 12 Jul 2022 16:35:41 +0200
From:   Harald Freudenberger <freude@linux.ibm.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux390-list@tuxmaker.boeblingen.de.ibm.com,
        linux-crypto@vger.kernel.org, linux-s390@vger.kernel.org,
        jchrist@linux.ibm.com, dengler@linux.ibm.com
Subject: Re: [PATCH] s390/archrandom: remove CPACF trng invocations in
 interrupt context
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On 2022-07-12 14:27, Jason A. Donenfeld wrote:
> Hi Harald,
> 
> On Tue, Jul 12, 2022 at 02:09:35PM +0200, Harald Freudenberger wrote:
>> > You've gone through the troubles of confirming experimentally what
>> > in_task() does, but that doesn't answer *why* it should be disallowed
>> > variously in each one of these contexts.
>> 
>> I think, I showed this. The only real occurrences remaining for the
>> arch_get_random_seed_long() call is within softirq context when the
>> network layer tries to allocate some skb buffers. My personal feeling
>> about this is that it does not hurt - but I asked our network guys
>> and their feedback is clear: no way - every delay there may cause
>> high bandwidth traffic to stumble and this is to be absolutely 
>> avoided.
>> However, they can't give me any measurements.
>> 
>> So yes, the intention is now with checking for in_task() to prevent
>> the trng call in hard and soft interrupt context. But still I'd like
>> to meet your condition to provide good random at kernel startup.
> 
> That's too bad, but okay.
> 
> Final question: do you see any of the in_task() vs in_whatever()
> semantics changing if arch_get_random_words{,_seed}() is ever
> implemented, which would reduce the current multitude of calls to the
> trng to a single call?
> 
> Jason

Hm, no, I can't see a way to provide trng random data in any whatever
interrupt context for the next future. The only enabler would be to
use a buffer ... I started to get in contact with our hardware guys
to make the trng data internally buffered and this the invocation
could be in no time give back random data. But this may be a
hardware development thing for the next machine generation.

Thanks Jason for your work

Regards
Harald Freudenberger