From patchwork Mon Mar 3 14:15:39 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Ryan Roberts X-Patchwork-Id: 13998913 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org 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 smtp.lore.kernel.org (Postfix) with ESMTPS id 12627C282D1 for ; Mon, 3 Mar 2025 14:16:36 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.900334.1308286 (Exim 4.92) (envelope-from ) id 1tp6aY-0005wJ-5n; Mon, 03 Mar 2025 14:16:10 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 900334.1308286; Mon, 03 Mar 2025 14:16:10 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1tp6aX-0005vw-Uw; Mon, 03 Mar 2025 14:16:09 +0000 Received: by outflank-mailman (input) for mailman id 900334; Mon, 03 Mar 2025 14:16:08 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1tp6aW-0005Dw-JC for xen-devel@lists.xenproject.org; Mon, 03 Mar 2025 14:16:08 +0000 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by se1-gles-flk1.inumbo.com (Halon) with ESMTP id 0c137a29-f83a-11ef-9898-31a8f345e629; Mon, 03 Mar 2025 15:16:06 +0100 (CET) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 4F7BE106F; Mon, 3 Mar 2025 06:16:20 -0800 (PST) Received: from e125769.cambridge.arm.com (e125769.cambridge.arm.com [10.1.196.27]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 2C3CA3F66E; Mon, 3 Mar 2025 06:16:04 -0800 (PST) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 0c137a29-f83a-11ef-9898-31a8f345e629 From: Ryan Roberts To: Andrew Morton , "David S. Miller" , Andreas Larsson , Juergen Gross , Boris Ostrovsky , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , "Matthew Wilcox (Oracle)" , Catalin Marinas Cc: Ryan Roberts , linux-mm@kvack.org, sparclinux@vger.kernel.org, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, David Hildenbrand Subject: [PATCH v2 5/5] Revert "x86/xen: allow nesting of same lazy mode" Date: Mon, 3 Mar 2025 14:15:39 +0000 Message-ID: <20250303141542.3371656-6-ryan.roberts@arm.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250303141542.3371656-1-ryan.roberts@arm.com> References: <20250303141542.3371656-1-ryan.roberts@arm.com> MIME-Version: 1.0 Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode") was added as a solution for a core-mm code change where arch_[enter|leave]_lazy_mmu_mode() started to be called in a nested manner; see commit bcc6cc832573 ("mm: add default definition of set_ptes()"). However, now that we have fixed the API to avoid nesting, we no longer need this capability in the x86 implementation. Additionally, from code review, I don't believe the fix was ever robust in the case of preemption occurring while in the nested lazy mode. The implementation usually deals with preemption by calling arch_leave_lazy_mmu_mode() from xen_start_context_switch() for the outgoing task if we are in the lazy mmu mode. Then in xen_end_context_switch(), it restarts the lazy mode by calling arch_enter_lazy_mmu_mode() for an incoming task that was in the lazy mode when it was switched out. But arch_leave_lazy_mmu_mode() will only unwind a single level of nesting. If we are in the double nest, then it's not fully unwound and per-cpu variables are left in a bad state. So the correct solution is to remove the possibility of nesting from the higher level (which has now been done) and remove this x86-specific solution. Acked-by: David Hildenbrand Signed-off-by: Ryan Roberts --- arch/x86/include/asm/xen/hypervisor.h | 15 ++------------- arch/x86/xen/enlighten_pv.c | 1 - 2 files changed, 2 insertions(+), 14 deletions(-) diff --git a/arch/x86/include/asm/xen/hypervisor.h b/arch/x86/include/asm/xen/hypervisor.h index a9088250770f..bd0fc69a10a7 100644 --- a/arch/x86/include/asm/xen/hypervisor.h +++ b/arch/x86/include/asm/xen/hypervisor.h @@ -72,18 +72,10 @@ enum xen_lazy_mode { }; DECLARE_PER_CPU(enum xen_lazy_mode, xen_lazy_mode); -DECLARE_PER_CPU(unsigned int, xen_lazy_nesting); static inline void enter_lazy(enum xen_lazy_mode mode) { - enum xen_lazy_mode old_mode = this_cpu_read(xen_lazy_mode); - - if (mode == old_mode) { - this_cpu_inc(xen_lazy_nesting); - return; - } - - BUG_ON(old_mode != XEN_LAZY_NONE); + BUG_ON(this_cpu_read(xen_lazy_mode) != XEN_LAZY_NONE); this_cpu_write(xen_lazy_mode, mode); } @@ -92,10 +84,7 @@ static inline void leave_lazy(enum xen_lazy_mode mode) { BUG_ON(this_cpu_read(xen_lazy_mode) != mode); - if (this_cpu_read(xen_lazy_nesting) == 0) - this_cpu_write(xen_lazy_mode, XEN_LAZY_NONE); - else - this_cpu_dec(xen_lazy_nesting); + this_cpu_write(xen_lazy_mode, XEN_LAZY_NONE); } enum xen_lazy_mode xen_get_lazy_mode(void); diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c index 5e57835e999d..919e4df9380b 100644 --- a/arch/x86/xen/enlighten_pv.c +++ b/arch/x86/xen/enlighten_pv.c @@ -99,7 +99,6 @@ struct tls_descs { }; DEFINE_PER_CPU(enum xen_lazy_mode, xen_lazy_mode) = XEN_LAZY_NONE; -DEFINE_PER_CPU(unsigned int, xen_lazy_nesting); enum xen_lazy_mode xen_get_lazy_mode(void) {