From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AH8x2253TLF0svgj9zq/ynsG5B6yUn2CWmRHNP7KzY9uAxDJyYjceIiEgNbXznj0ttu5TY8QoybB ARC-Seal: i=1; a=rsa-sha256; t=1517249826; cv=none; d=google.com; s=arc-20160816; b=z5/2ri6pq0MKllRQFmFi+FqoRqJZ4BknSG8JhQ0HQHQs+AumRJ2RqhfH2bwjqaGCnl iStaQ5S/0siSNpWP9Mxu5i3dRaZXJ/EOeRWS+vZUAkTgNIWOOoMNOs01OwVqB1fS8Gus SKIRfim8nwj8eJVMyQ7ui2LBleFcRJBINTGBywqiNDwfIvjgLV6Re6XbRoBuVMAdduxF qiVP79Hcl5aEwDYQvUvMUedOFi+UuzNlTRyvwfItxglJr9JgSYcPL8l77YBlLKGIer3/ GNQSQWvn6ZEXKzSEo+bRyMrhxiUpYBABXzxzDdeqAaFOZxQudrIvaOhHs+ZqOWrK7O4b Gd3g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:content-transfer-encoding :content-language:in-reply-to:mime-version:user-agent:date :message-id:from:references:cc:to:subject:arc-authentication-results; bh=9caV2913+LFIFZfxnf8LZiBIr9urtnV9ctql5oE7t+Y=; b=oJ0VBDVoC9WM5psMP8xYgGu9PVjNVc1FzE8+Fo9QEKL3JWeAMmhwMF/XeJs5EBsjcN yJLMc8a9hCOquZE6D4u/C/RE4jdWv9nyMRLfXZbfbKH3gc54Wy61jl1cuBLAdToTbydD lcs/F960AP8vvWnDtnJnWHiXrOYaPU35RAaOnuH4YXTWuI0rLQk0IxVl80xjqTnzEcJM dyCaaIOIHJc2xC/B0g8HvkEtyb1wW22MphRHrhv1OsawOed51rF5Nc4TgUk0qlPQM4bc wp6K5NosospquI9ry3PPOPmsq74qfyBimefRjejRPMMWCIqzvfZNHLqdvYrn7muIp1mN 4K3w== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kselftest-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kselftest-owner@vger.kernel.org Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kselftest-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kselftest-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751273AbeA2SRG (ORCPT ); Mon, 29 Jan 2018 13:17:06 -0500 Received: from terminus.zytor.com ([65.50.211.136]:42159 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751072AbeA2SRF (ORCPT ); Mon, 29 Jan 2018 13:17:05 -0500 Subject: Re: selftests/x86/fsgsbase_64 test problem To: Andy Lutomirski Cc: Borislav Petkov , Dan Rue , Shuah Khan , Ingo Molnar , Dmitry Safonov , "open list:KERNEL SELFTEST FRAMEWORK" , LKML References: <20180126153631.ha7yc33fj5uhitjo@xps> <46328204-e363-e517-f30c-c8c94ac1442c@zytor.com> From: "H. Peter Anvin" Message-ID: <624b9e67-14ee-6882-e55e-f337ec2471d3@zytor.com> Date: Mon, 29 Jan 2018 10:12:32 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kselftest-owner@vger.kernel.org X-Mailing-List: linux-kselftest@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1590669876861238825?= X-GMAIL-MSGID: =?utf-8?q?1590951754642941919?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 01/29/18 08:37, Andy Lutomirski wrote: > > That's what I thought, too, and the SDM does say that, but the SDM > says all kinds of not-quite-correct things about segmentation. > >> It is pretty much scratch space (I have >> suggested using it for the gsbase once all those issues get sorted out, >> because it lets the paranoid code do something like: >> >> rdgsbase %rax >> push %rax /* Save old gsbase */ >> push %rax /* Reserve space on stack */ >> sgdt -2(%rsp) /* We don't care about the limit */ >> pop %rax /* %rax <- gdtbase */ >> mov (%rax),%rax /* GDT[0] holds the gsbase for this cpu */ >> wrgsbase %rax > > That will utterly suck on non-UMIP machines that have > hypervisor-provided UMIP emulation. > Is that a valid thing to optimize for, especially given that paranoid entries aren't the most common anyway? -hpa