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,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 6CBF1C2BA83 for ; Fri, 7 Feb 2020 19:45:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4AC9320726 for ; Fri, 7 Feb 2020 19:45:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727195AbgBGTpd (ORCPT ); Fri, 7 Feb 2020 14:45:33 -0500 Received: from mga01.intel.com ([192.55.52.88]:21548 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726900AbgBGTpd (ORCPT ); Fri, 7 Feb 2020 14:45:33 -0500 X-Amp-Result: UNKNOWN X-Amp-Original-Verdict: FILE UNKNOWN X-Amp-File-Uploaded: False Received: from orsmga001.jf.intel.com ([10.7.209.18]) by fmsmga101.fm.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 07 Feb 2020 11:45:32 -0800 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.70,414,1574150400"; d="scan'208";a="312123734" Received: from sjchrist-coffee.jf.intel.com (HELO linux.intel.com) ([10.54.74.202]) by orsmga001.jf.intel.com with ESMTP; 07 Feb 2020 11:45:32 -0800 Date: Fri, 7 Feb 2020 11:45:32 -0800 From: Sean Christopherson To: Peter Xu , Vitaly Kuznetsov Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Christoffer Dall , Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Message-ID: <20200207194532.GK2401@linux.intel.com> References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-16-sean.j.christopherson@intel.com> <20200206200200.GC700495@xz-x1> <20200206212120.GF13067@linux.intel.com> <20200206214106.GG700495@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200206214106.GG700495@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org +Vitaly for HyperV On Thu, Feb 06, 2020 at 04:41:06PM -0500, Peter Xu wrote: > On Thu, Feb 06, 2020 at 01:21:20PM -0800, Sean Christopherson wrote: > > On Thu, Feb 06, 2020 at 03:02:00PM -0500, Peter Xu wrote: > > > But that matters to this patch because if MIPS can use > > > kvm_flush_remote_tlbs(), then we probably don't need this > > > arch-specific hook any more and we can directly call > > > kvm_flush_remote_tlbs() after sync dirty log when flush==true. > > > > Ya, the asid_flush_mask in kvm_vz_flush_shadow_all() is the only thing > > that prevents calling kvm_flush_remote_tlbs() directly, but I have no > > clue as to the important of that code. > > As said above I think the x86 lockdep is really not necessary, then > considering MIPS could be the only one that will use the new hook > introduced in this patch... Shall we figure that out first? So I prepped a follow-up patch to make kvm_arch_dirty_log_tlb_flush() a MIPS-only hook and use kvm_flush_remote_tlbs() directly for arm and x86, but then I realized x86 *has* a hook to do a precise remote TLB flush. There's even an existing kvm_flush_remote_tlbs_with_address() call on a memslot, i.e. this exact scenario. So arguably, x86 should be using the more precise flush and should keep kvm_arch_dirty_log_tlb_flush(). But, the hook is only used when KVM is running as an L1 on top of HyperV, and I assume dirty logging isn't used much, if at all, for L1 KVM on HyperV? I see three options: 1. Make kvm_arch_dirty_log_tlb_flush() MIPS-only and call kvm_flush_remote_tlbs() directly for arm and x86. Add comments to explain when an arch should implement kvm_arch_dirty_log_tlb_flush(). 2. Change x86 to use kvm_flush_remote_tlbs_with_address() when flushing a memslot after the dirty log is grabbed by userspace. 3. Keep the resulting code as is, but add a comment in x86's kvm_arch_dirty_log_tlb_flush() to explain why it uses kvm_flush_remote_tlbs() instead of the with_address() variant. I strongly prefer to (2) or (3), but I'll defer to Vitaly as to which of those is preferable. I don't like (1) because (a) it requires more lines code (well comments), to explain why kvm_flush_remote_tlbs() is the default, and (b) it would require even more comments, which would be x86-specific in generic KVM, to explain why x86 doesn't use its with_address() flush, or we'd lost that info altogether.