Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Gabriel Paubert <paubert@iram.es>
Cc:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        x86@kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        Harald Freudenberger <freude@linux.ibm.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Borislav Petkov <bp@suse.de>, Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v2] random: handle archrandom in plural words
In-Reply-To: <YtUB3Iola9R0OwN6@lt-gp.iram.es>
References: <YtP1+MJ1tNdJA60l@zx2c4.com>
 <20220717200356.75060-1-Jason@zx2c4.com>
 <87a697dj9s.fsf@mpe.ellerman.id.au> <YtUB3Iola9R0OwN6@lt-gp.iram.es>
Date:   Wed, 20 Jul 2022 13:03:19 +1000
Message-ID: <87y1wocwp4.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Gabriel Paubert <paubert@iram.es> writes:
> On Mon, Jul 18, 2022 at 04:31:11PM +1000, Michael Ellerman wrote:
>> "Jason A. Donenfeld" <Jason@zx2c4.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.

Yeah I should have said on *some* other 64-bit arches.

Seems to be a common feature/hack on arches that have evolved over time,
or been inspired by earlier arches.

The latest Power ISA has octwords :)

cheers