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Ts'o" Cc: linux-kernel@vger.kernel.org, Arnd Bergmann , Greg Kroah-Hartman , Herbert Xu Subject: Re: [PATCH] random: Drop ARCH limitations for CONFIG_RANDOM_TRUST_CPU Message-Id: <20200329202624.087251ba5c6d2d9715e5710f@gmail.com> In-Reply-To: <20200329165624.GO53396@mit.edu> References: <20200329082909.193910-1-alexander.sverdlin@gmail.com> <20200329165624.GO53396@mit.edu> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello Ted, On Sun, 29 Mar 2020 12:56:24 -0400 "Theodore Y. Ts'o" wrote: > On Sun, Mar 29, 2020 at 10:29:09AM +0200, Alexander Sverdlin wrote: > > The option itself looks attractive for the embedded devices which often > > have HWRNG but less entropy from user-input. And these devices are often > > ARM/ARM64 or MIPS. The reason to limit it to X86/S390/PPC is not obvious. > > > > Signed-off-by: Alexander Sverdlin > > This feature is *only* applicable if the CPU supports a > arch_get_random_seed_long() or arch_get_random_long(). I believe > there are some server-class ARM64 CPU's that support such an > instruction, but I don't believe any of the embedded arm64 --- and > certainly non of the embedded arm --- SOC's support > arch_get_random_long(). you are right! Thank you for the explanation! I totally missed the fact that it's not connected to hwrng drivers... Please ignore the patch. [...] > So we should either add ARM64 to the dependency list, or we could, as > you suggest, simply remove the dependency altogether. The tradeoff is > that it will cause an extra CONFIG prompt on a number of platforms > (mips, arm, sparc, etc.) where it will be utterly pointless since > those architectures have no chance of support a RDRAND-like > instruction. -- Alexander Sverdlin.