From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DCED3C433E0 for ; Wed, 27 May 2020 15:39:32 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id AFA8D20776 for ; Wed, 27 May 2020 15:39:32 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org AFA8D20776 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=citrix.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jdy93-0000i5-Pd; Wed, 27 May 2020 15:39:05 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jdy92-0000hv-2F for xen-devel@lists.xenproject.org; Wed, 27 May 2020 15:39:04 +0000 X-Inumbo-ID: 307b211d-a030-11ea-a760-12813bfff9fa Received: from ppsw-31.csi.cam.ac.uk (unknown [131.111.8.131]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 307b211d-a030-11ea-a760-12813bfff9fa; Wed, 27 May 2020 15:39:03 +0000 (UTC) X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: http://help.uis.cam.ac.uk/email-scanner-virus Received: from 88-109-182-220.dynamic.dsl.as9105.com ([88.109.182.220]:37566 helo=[192.168.1.219]) by ppsw-31.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.157]:465) with esmtpsa (PLAIN:amc96) (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128) id 1jdy8y-000MCH-KJ (Exim 4.92.3) (return-path ); Wed, 27 May 2020 16:39:00 +0100 Subject: Re: [PATCH] x86/boot: Fix load_system_tables() to be NMI/#MC-safe To: Jan Beulich References: <20200527130607.32069-1-andrew.cooper3@citrix.com> <50f66504-ab7b-2f3e-1695-003ad69ae37a@suse.com> From: Andrew Cooper Message-ID: Date: Wed, 27 May 2020 16:38:59 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <50f66504-ab7b-2f3e-1695-003ad69ae37a@suse.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-GB X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Xen-devel , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 27/05/2020 14:19, Jan Beulich wrote: > On 27.05.2020 15:06, Andrew Cooper wrote: >> @@ -720,30 +721,26 @@ void load_system_tables(void) >> .limit = (IDT_ENTRIES * sizeof(idt_entry_t)) - 1, >> }; >> >> - *tss = (struct tss64){ >> - /* Main stack for interrupts/exceptions. */ >> - .rsp0 = stack_bottom, >> - >> - /* Ring 1 and 2 stacks poisoned. */ >> - .rsp1 = 0x8600111111111111ul, >> - .rsp2 = 0x8600111111111111ul, >> - >> - /* >> - * MCE, NMI and Double Fault handlers get their own stacks. >> - * All others poisoned. >> - */ >> - .ist = { >> - [IST_MCE - 1] = stack_top + IST_MCE * PAGE_SIZE, >> - [IST_DF - 1] = stack_top + IST_DF * PAGE_SIZE, >> - [IST_NMI - 1] = stack_top + IST_NMI * PAGE_SIZE, >> - [IST_DB - 1] = stack_top + IST_DB * PAGE_SIZE, >> - >> - [IST_MAX ... ARRAY_SIZE(tss->ist) - 1] = >> - 0x8600111111111111ul, >> - }, >> - >> - .bitmap = IOBMP_INVALID_OFFSET, >> - }; >> + /* >> + * Set up the TSS. Warning - may be live, and the NMI/#MC must remain >> + * valid on every instruction boundary. (Note: these are all >> + * semantically ACCESS_ONCE() due to tss's volatile qualifier.) >> + * >> + * rsp0 refers to the primary stack. #MC, #DF, NMI and #DB handlers >> + * each get their own stacks. No IO Bitmap. >> + */ >> + tss->rsp0 = stack_bottom; >> + tss->ist[IST_MCE - 1] = stack_top + IST_MCE * PAGE_SIZE; >> + tss->ist[IST_DF - 1] = stack_top + IST_DF * PAGE_SIZE; >> + tss->ist[IST_NMI - 1] = stack_top + IST_NMI * PAGE_SIZE; >> + tss->ist[IST_DB - 1] = stack_top + IST_DB * PAGE_SIZE; >> + tss->bitmap = IOBMP_INVALID_OFFSET; >> + >> + /* All other stack pointers poisioned. */ >> + for ( i = IST_MAX; i < ARRAY_SIZE(tss->ist); ++i ) >> + tss->ist[i] = 0x8600111111111111ul; >> + tss->rsp1 = 0x8600111111111111ul; >> + tss->rsp2 = 0x8600111111111111ul; > ACCESS_ONCE() unfortunately only has one of the two needed effects: > It guarantees that each memory location gets accessed exactly once > (which I assume can also be had with just the volatile addition, > but without the moving away from using an initializer), but it does > not guarantee single-insn accesses. Linux's memory-barriers.txt disagrees, and specifically gives an example with a misaligned int (vs two shorts) and the use volatile cast (by way of {READ,WRITE}_ONCE()) to prevent load/store tearing, as the memory location is of a size which can be fit in a single access. I'm fairly sure we're safe here. > I consider this in particular > relevant here because all of the 64-bit fields are misaligned. By > doing it like you do, we're setting us up to have to re-do this yet > again in a couple of years time (presumably using write_atomic() > instead then). > > Nevertheless it is a clear improvement, so if you want to leave it > like this > Reviewed-by: Jan Beulich Thanks, ~Andrew