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[209.132.180.67]) by mx.google.com with ESMTP id 13si9439848itx.111.2018.01.23.14.38.31; Tue, 23 Jan 2018 14:38:54 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@amdcloud.onmicrosoft.com header.s=selector1-amd-com header.b=EfWMr2mS; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932336AbeAWWh6 (ORCPT + 99 others); Tue, 23 Jan 2018 17:37:58 -0500 Received: from mail-sn1nam02on0053.outbound.protection.outlook.com ([104.47.36.53]:21408 "EHLO NAM02-SN1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932291AbeAWWh4 (ORCPT ); Tue, 23 Jan 2018 17:37:56 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amdcloud.onmicrosoft.com; s=selector1-amd-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version; bh=aU8WEbahsUQTBFeRLpBtJFFdzcDUi5Y1dGMhI6SL8dM=; b=EfWMr2mSgkFrZNv37jz6VGx7AzugT7YSLQSSSCnpI+SndbyZ0GJx7eAZYTAOzZNRewC6zcJtB4KMRk4T0fkF8nDUYhlqUaJ5Ijng+TR6n8xoXq4UaE5zhRwVAE9Jt1uBtZHwaTG4Mi42gV0ATtihZNoUH6PkFDy0dSlCK/1nDeA= Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Thomas.Lendacky@amd.com; Received: from [10.236.65.116] (165.204.78.1) by MWHPR12MB1152.namprd12.prod.outlook.com (10.169.204.16) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P256) id 15.20.428.17; Tue, 23 Jan 2018 22:37:45 +0000 Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation To: "Woodhouse, David" , Andy Lutomirski , KarimAllah Ahmed Cc: linux-kernel@vger.kernel.org, Andi Kleen , Andrea Arcangeli , Andy Lutomirski , Arjan van de Ven , Ashok Raj , Asit Mallick , Borislav Petkov , Dan Williams , Dave Hansen , Greg Kroah-Hartman , "H . Peter Anvin" , Ingo Molnar , Janakarajan Natarajan , Joerg Roedel , Jun Nakajima , Laura Abbott , Linus Torvalds , Masami Hiramatsu , Paolo Bonzini , Peter Zijlstra , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Thomas Gleixner , Tim Chen , kvm@vger.kernel.org, x86@kernel.org, Arjan Van De Ven References: <1516476182-5153-1-git-send-email-karahmed@amazon.de> <1516476182-5153-10-git-send-email-karahmed@amazon.de> <243BE571-AF73-44B3-8D17-193F9E07686A@amacapital.net> <4e01a7a9-29e4-adcc-3f53-550fb7f3d370@amd.com> <1516724457.9521.156.camel@amazon.co.uk> From: Tom Lendacky Message-ID: Date: Tue, 23 Jan 2018 16:37:35 -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: <1516724457.9521.156.camel@amazon.co.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-Originating-IP: [165.204.78.1] X-ClientProxiedBy: BN6PR1101CA0018.namprd11.prod.outlook.com (10.174.237.28) To 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X-Microsoft-Exchange-Diagnostics: 1;MWHPR12MB1152;20:dbE3JpkbSG+spXnhZtOcXHUFD/o6JAeRyMlkvuhbcBFkMIiJum6mvUwf6XU/Ykz/DMd6noySaCGa+rfUaVih3qrCrT1KjB4p4XBrGgRSXqrU4FJTSZVlKlrDAPyDZpjVxd0xH+W7UsFYqtmVACbSELF263aoM6/EDSeiEnXFdEmGkkJ7x9zEOsEYCnfI1HQAMAAfqsv+r/nLq3EK6Mwrr11lhuiMXgPNFsP8AaXHyPAX1m71MfhgCR/eB+XHu6HA X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Jan 2018 22:37:45.4352 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d06ef053-c288-429f-0bc1-08d562b1ee9b 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/23/2018 10:20 AM, Woodhouse, David wrote: > On Tue, 2018-01-23 at 10:12 -0600, Tom Lendacky wrote: >> >>>> +.macro UNRESTRICT_IB_SPEC >>>> +    ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_IBRS >>>> +    PUSH_MSR_REGS >>>> +    WRMSR_ASM $MSR_IA32_SPEC_CTRL, $0, $0 >>>   >> I think you should be writing 2, not 0, since I'm reasonably >> confident that we want STIBP on.  Can you explain why you're writing >> 0? >> >> Do we want to talk about STIBP in general?  Should it be (yet another) >> boot option to enable or disable?  If there is STIBP support without >> IBRS support, it could be a set and forget at boot time. > > We haven't got patches which enable STIBP in general. The kernel itself > is safe either way with retpoline, or because IBRS implies STIBP too > (that is, there's no difference between writing 1 and 3). > > So STIBP is purely about protecting userspace processes from one > another, and VM guests from one another, when they run on HT siblings. > > There's an argument that there are so many other information leaks > between HT siblings that we might not care. Especially as it's hard to > *tell* when you're scheduling, whether you trust all the processes (or > guests) on your HT siblings right now... let alone later when > scheduling another process if you need to *now* set STIBP on a sibling > which is no longer save from this process now running. > > I'm not sure we want to set STIBP *unconditionally* either because of > the performance implications. > > For IBRS we had an answer and it was just ugly. For STIBP we don't > actually have an answer for "how do we use this?". Do we? Not sure. Maybe to start, the answer might be to allow it to be set for the ultra-paranoid, but in general don't enable it by default. Having it enabled would be an alternative to someone deciding to disable SMT, since that would have even more of a performance impact. Thanks, Tom > >