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Fri, 07 Feb 2020 16:18:37 -0800 (PST) X-Google-Smtp-Source: APXvYqzAkXLXCBlBGfr3jniMMbcCpWv44CMlkznbhHCioVsTu+8iTId/2cuSvhY/ok33KerrRsEvMw== X-Received: by 2002:a37:8cc:: with SMTP id 195mr1462239qki.456.1581121116707; Fri, 07 Feb 2020 16:18:36 -0800 (PST) Received: from xz-x1 ([2607:9880:19c8:32::2]) by smtp.gmail.com with ESMTPSA id n32sm2271790qtk.66.2020.02.07.16.18.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Feb 2020 16:18:35 -0800 (PST) Date: Fri, 7 Feb 2020 19:18:32 -0500 From: Peter Xu To: Sean Christopherson Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Message-ID: <20200208001832.GA823968@xz-x1> 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> <20200207194532.GK2401@linux.intel.com> MIME-Version: 1.0 In-Reply-To: <20200207194532.GK2401@linux.intel.com> X-MC-Unique: Hk6vISYXMgypOHpERGk0Wg-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200207_161843_601676_24A1ED36 X-CRM114-Status: GOOD ( 22.14 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Wanpeng Li , kvm@vger.kernel.org, David Hildenbrand , linux-mips@vger.kernel.org, Paul Mackerras , kvmarm@lists.cs.columbia.edu, Janosch Frank , Marc Zyngier , Joerg Roedel , Christian Borntraeger , Julien Thierry , Suzuki K Poulose , kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Jim Mattson , Cornelia Huck , Christoffer Dall , linux-kernel@vger.kernel.org, James Morse , Paolo Bonzini , Vitaly Kuznetsov , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Feb 07, 2020 at 11:45:32AM -0800, Sean Christopherson wrote: > +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. > I proposed the 4th solution here: https://lore.kernel.org/kvm/20200207223520.735523-1-peterx@redhat.com/ I'm not sure whether that's acceptable, but if it can, then we can drop the kvm_arch_dirty_log_tlb_flush() hook, or even move on to per-slot tlb flushing. Thanks, -- Peter Xu _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel