linux-kernel - Re: [PATCH v2] random: handle archrandom in plural words

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Message-ID: <YtUB3Iola9R0OwN6@lt-gp.iram.es>
Date:   Mon, 18 Jul 2022 08:46:52 +0200
From:   Gabriel Paubert <paubert@...m.es>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        x86@...nel.org, Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Johannes Berg <johannes@...solutions.net>,
        Harald Freudenberger <freude@...ux.ibm.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Borislav Petkov <bp@...e.de>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] random: handle archrandom in plural words

On Mon, Jul 18, 2022 at 04:31:11PM +1000, Michael Ellerman wrote:
> "Jason A. Donenfeld" <Jason@...c4.com> writes:
> > The archrandom interface was originally designed for x86, which supplies
> > RDRAND/RDSEED for receiving random words into registers, resulting in
> > one function to generate an int and another to generate a long. However,
> > other architectures don't follow this.
> >
> > On arm64, the SMCCC TRNG interface can return between 1 and 3 words. On
> > s390, the CPACF TRNG interface can return between 1 and 32 words for the
> > same cost as for one word. On UML, the os_getrandom() interface can return
> > arbitrary amounts.
> >
> > So change the api signature to take a "words" parameter designating the
> > maximum number of words requested, and then return the number of words
> > generated.
> 
> On powerpc a word is 32-bits and a doubleword is 64-bits (at least
> according to the ISA). I think that's also true on other 64-bit
> architectures.

IIRC, this is (or was) not the case on Alpha, where word was defined as
16 bits. All assembly mnemonics had w for 16 bits, l for 32 bits, and q
for 64 bits.

Blame the PDP-11...

	Gabriel

> 
> You could avoid any confusion by defining the API in terms of "longs"
> rather than "words".
> 
> But that's just a comment, see what others think.
> 
> >  arch/powerpc/include/asm/archrandom.h |  30 ++------
> >  arch/powerpc/kvm/book3s_hv.c          |   2 +-
> 
> Acked-by: Michael Ellerman <mpe@...erman.id.au> (powerpc)
> 
> cheers