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Date:   Tue, 12 Jul 2022 16:44:30 +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 04:35:41PM +0200, Harald Freudenberger wrote:
> 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.

Alrightie then. Well, I'll Ack a v2 that keeps the _int variant. Sounds
like then we'll be done here.

Tangential topic: would be nice to add the TRNG instruction to QEMU's
TCG so that VMs in CI have a bit of randomness available.

Jason