Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753443AbbGaP2V (ORCPT ); Fri, 31 Jul 2015 11:28:21 -0400 Received: from mail-qk0-f182.google.com ([209.85.220.182]:35453 "EHLO mail-qk0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751369AbbGaP2T (ORCPT ); Fri, 31 Jul 2015 11:28:19 -0400 From: Len Brown To: x86@kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Len Brown Subject: [PATCH 1/1] x86: replace RDRAND forced-reseed with simple sanity check Date: Fri, 31 Jul 2015 11:27:39 -0400 Message-Id: <001e354e9435443b1720838a111620c4eec12a61.1438356386.git.len.brown@intel.com> X-Mailer: git-send-email 2.5.0 Reply-To: Len Brown Organization: Intel Open Source Technology Center Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2909 Lines: 87 From: Len Brown x86_init_rdrand() was added with 2 goals: 1. Sanity check that the built-in-self-test circuit on the Digital Random Number Generator (DRNG) is not complaining. As RDRAND HW self-checks on every invocation, this goal is achieved by simply invoking RDRAND and checking its return code. 2. Force a full re-seed of the random number generator. This was done out of paranoia to benefit the most un-sophisticated DRNG implementation conceivable in the architecture, an implementation that does not exist, and unlikely ever will. This worst-case full-re-seed is achieved by invoking a 64-bit RDRAND 8192 times. Unfortunately, this worst-case re-seed costs O(1,000us). Magnifying this cost, it is done from identify_cpu(), which is the synchronous critical path to bring a processor on-line -- repeated for every logical processor in the system at boot and resume from S3. As it is very expensive, and of highly dubious value, we delete the worst-case re-seed from the kernel. We keep the 1st goal -- sanity check the hardware, and mark it absent if it complains. This change reduces the cost of x86_init_rdrand() by a factor of 1,000x, to O(1us) from O(1,000us). Signed-off-by: Len Brown --- arch/x86/kernel/cpu/rdrand.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-) diff --git a/arch/x86/kernel/cpu/rdrand.c b/arch/x86/kernel/cpu/rdrand.c index 136ac74..b86817e 100644 --- a/arch/x86/kernel/cpu/rdrand.c +++ b/arch/x86/kernel/cpu/rdrand.c @@ -33,28 +33,26 @@ static int __init x86_rdrand_setup(char *s) __setup("nordrand", x86_rdrand_setup); /* - * Force a reseed cycle; we are architecturally guaranteed a reseed - * after no more than 512 128-bit chunks of random data. This also - * acts as a test of the CPU capability. + * RDRAND has Built-In-Self-Test (BIST) that runs on every invocation. + * Run the instruction a few times as a sanity check. + * If it fails, it is simple to disable RDRAND here. */ -#define RESEED_LOOP ((512*128)/sizeof(unsigned long)) +#define SANITY_CHECK_LOOPS 8 void x86_init_rdrand(struct cpuinfo_x86 *c) { #ifdef CONFIG_ARCH_RANDOM unsigned long tmp; - int i, count, ok; + int i; if (!cpu_has(c, X86_FEATURE_RDRAND)) - return; /* Nothing to do */ + return; - for (count = i = 0; i < RESEED_LOOP; i++) { - ok = rdrand_long(&tmp); - if (ok) - count++; + for (i = 0; i < SANITY_CHECK_LOOPS; i++) { + if (!rdrand_long(&tmp)) { + clear_cpu_cap(c, X86_FEATURE_RDRAND); + return; + } } - - if (count != RESEED_LOOP) - clear_cpu_cap(c, X86_FEATURE_RDRAND); #endif } -- 2.5.0 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/