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Date:   Tue, 12 Jul 2022 14:27:00 +0200
From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     Harald Freudenberger <freude@linux.ibm.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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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