From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933715AbeAHOr0 (ORCPT + 1 other); Mon, 8 Jan 2018 09:47:26 -0500 Received: from mail-cys01nam02on0070.outbound.protection.outlook.com ([104.47.37.70]:21776 "EHLO NAM02-CY1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751597AbeAHOrY (ORCPT ); Mon, 8 Jan 2018 09:47:24 -0500 Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Thomas.Lendacky@amd.com; Subject: Re: [tip:x86/pti] x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC To: Thomas Gleixner , Andrew Cooper Cc: bp@alien8.de, dwmw@amazon.co.uk, gregkh@linux-foundation.org, pjt@google.com, mingo@kernel.org, linux-kernel@vger.kernel.org, hpa@zytor.com, tim.c.chen@linux.intel.com, torvalds@linux-foundation.org, peterz@infradead.org, dave.hansen@intel.com, linux-tip-commits@vger.kernel.org References: <20180105160756.23786.4220.stgit@tlendack-t1.amdoffice.net> <1b179b8b-6cc8-d736-81dc-2445be4baf02@citrix.com> From: Tom Lendacky Message-ID: <4d9a1f0d-a401-3f0f-9ee2-dd42f4b4716a@amd.com> Date: Mon, 8 Jan 2018 08:47:15 -0600 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [165.204.78.1] X-ClientProxiedBy: MWHPR1601CA0018.namprd16.prod.outlook.com (10.172.93.28) To MWHPR12MB1152.namprd12.prod.outlook.com (10.169.204.16) X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-HT: Tenant X-MS-Office365-Filtering-Correlation-Id: a087c8b9-f542-44ac-03cb-08d556a6b948 X-Microsoft-Antispam: UriScan:;BCL:0;PCL:0;RULEID:(48565401081)(4534020)(4602075)(4627115)(201703031133081)(201702281549075)(5600026)(4604075)(2017052603307)(7153060)(7193020);SRVR:MWHPR12MB1152; X-Microsoft-Exchange-Diagnostics: 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X-Microsoft-Exchange-Diagnostics: 1;MWHPR12MB1152;20:M7a/MR1pXsPjn0YuVHDhmyKZ++wOl4fIwWGrKn3UXvaEicerGNayp1yqf89t062HWGeycQevFwjkk6wYWnwwWmaMMc8Vz8cnv6v3iat6AFnXJbnkfUKfYX8ejrM34NE0JxasXUQEW9wgn/aQLQNV63SerdbFPguNpI8J3FjHi99dgoeOOX9uAMl4B8yE439cSbHJJJLB5mgUHktGf6vLIAiefx/5Alio5XOm1kvqnli4f3Nr7tWBNbtwcyXoojIl X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 08 Jan 2018 14:47:20.3045 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: a087c8b9-f542-44ac-03cb-08d556a6b948 X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-Transport-CrossTenantHeadersStamped: MWHPR12MB1152 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: On 1/8/2018 5:10 AM, Thomas Gleixner wrote: > On Mon, 8 Jan 2018, Andrew Cooper wrote: > >> On 08/01/18 10:08, Thomas Gleixner wrote: >>> On Sat, 6 Jan 2018, tip-bot for Tom Lendacky wrote: >>> >>>> Commit-ID: 0bf17c102177d5da9363bf8b1e4704b9996d5079 >>>> Gitweb: https://git.kernel.org/tip/0bf17c102177d5da9363bf8b1e4704b9996d5079 >>>> Author: Tom Lendacky >>>> AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600 >>>> Committer: Thomas Gleixner >>>> CommitDate: Sat, 6 Jan 2018 21:57:40 +0100 >>>> >>>> x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC >>>> >>>> With LFENCE now a serializing instruction, set the LFENCE_RDTSC >>>> feature since the LFENCE instruction has less overhead than the >>>> MFENCE instruction. >>> Second thoughts on that. As pointed out by someone in one of the insane >>> long threads: >>> >>> What happens if the kernel runs as a guest and >>> >>> - the hypervisor did not set the LFENCE to serializing on the host >>> >>> - the hypervisor does not allow writing MSR_AMD64_DE_CFG >>> >>> That would bring the guest into a pretty bad state or am I missing >>> something essential here? >> >> What I did in Xen was to attempt to set it, then read it back and see.  >> If LFENCE still isn't serialising, using repoline is the only available >> mitigation. >> >> My understanding from the folk at AMD is that retpoline is safe to use, >> but has higher overhead than the LFENCE approach. Correct, the retpoline will work, it just takes more cycles. > > That still does not help vs. rdtsc_ordered() and LFENCE_RDTSC ... Ok, I can add the read-back check before setting the feature flag(s). But... what about the case where the guest is a different family than hypervisor? If we're on, say, a Fam15h hypervisor but the guest is started as a Fam0fh guest where the MSR doesn't exist and LFENCE is supposed to be serialized? I'll have to do a rdmsr_safe() and only set the flag(s) if I can successfully read the MSR back and validate the bit. Thanks, Tom > > Thanks, > > tglx >