linux-kernel - Re: [v3 PATCH] hwrng: core - Remove add_early

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Date: Thu, 23 May 2024 12:49:50 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Nícolas F. R. A. Prado <nfraprado@...labora.com>,
	Eric Biggers <ebiggers@...nel.org>,
	Jarkko Sakkinen <jarkko@...nel.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
	linux-integrity@...r.kernel.org, keyrings@...r.kernel.org,
	regressions@...ts.linux.dev, kernel@...labora.com,
	Tejun Heo <tj@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>, Torsten Duwe <duwe@....de>,
	"H. Peter Anvin" <hpa@...or.com>, Theodore Ts'o <tytso@....edu>,
	"Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [v3 PATCH] hwrng: core - Remove add_early_randomness

On Wed, May 22, 2024 at 03:53:23PM -0700, Linus Torvalds wrote:
> 
> That said, looking at the code in question, there are other oddities
> going on. Even the "we found a favorite new rng" case looks rather
> strange. The thread we use - nice and asynchronous - seems to sleep
> only if the randomness source is emptied.
> 
> What if you have a really good source of hw randomness? That looks
> like a busy loop to me, but hopefully I'm missing something obvious.

Yes that does look strange.  So I dug up the original patch at

	https://lore.kernel.org/all/20140317165012.GC1763@lst.de/

and therein lies the answer.  It's relying on random.c to push back
when the amount of new entropy exceeds what it needs.  IOW we will
sleep via add_hwgenerator_randomness when random.c decides that
enough is enough.  In fact the rate is much less now compared to
when the patch was first applied.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt