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Fri, 11 Dec 2020 07:25:20 -0800 (PST) X-Google-Smtp-Source: ABdhPJxp9yYldwC0MGQIrT+t7qi9g6CDUEkHB0XegmFIPjn12FW5NuZxMBXj4GhD/nDOrrmsxlvMBg== X-Received: by 2002:a37:4384:: with SMTP id q126mr16863492qka.30.1607700320477; Fri, 11 Dec 2020 07:25:20 -0800 (PST) Received: from xz-x1 ([142.126.83.202]) by smtp.gmail.com with ESMTPSA id u22sm6854022qkk.51.2020.12.11.07.25.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Dec 2020 07:25:19 -0800 (PST) Date: Fri, 11 Dec 2020 10:25:18 -0500 From: Peter Xu To: zhukeqian Subject: Re: [PATCH] kvm: Take into account the unaligned section size when preparing bitmap Message-ID: <20201211152518.GD6520@xz-x1> References: <20201208114013.875-1-yuzenghui@huawei.com> <20201208151654.GA6432@xz-x1> <6dc82702-9246-4684-4f28-e104abc0c11d@huawei.com> <20201210020843.GB3211@xz-x1> <7d46e5ca-24ab-7c44-201c-77e8fc6a2ace@huawei.com> <20201210145006.GD3211@xz-x1> <2607b4cd-524c-2360-6261-224736861fc4@huawei.com> MIME-Version: 1.0 In-Reply-To: <2607b4cd-524c-2360-6261-224736861fc4@huawei.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=peterx@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Received-SPF: pass client-ip=63.128.21.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Zenghui Yu , pbonzini@redhat.com, qemu-devel@nongnu.org, wanghaibin.wang@huawei.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Dec 11, 2020 at 09:13:10AM +0800, zhukeqian wrote: > > On 2020/12/10 22:50, Peter Xu wrote: > > On Thu, Dec 10, 2020 at 10:53:23AM +0800, zhukeqian wrote: > >> > >> > >> On 2020/12/10 10:08, Peter Xu wrote: > >>> Keqian, > >>> > >>> On Thu, Dec 10, 2020 at 09:46:06AM +0800, zhukeqian wrote: > >>>> Hi, > >>>> > >>>> I see that if start or size is not PAGE aligned, it also clears areas > >>>> which beyond caller's expectation, so do we also need to consider this? > >>> > >>> Could you elaborate? > >>> > >>> If start_delta != 0, kvm_log_clear_one_slot() should already go the slow path. > >>> > >>> Thanks, > >>> > >> > >> Hi Peter, > >> > >> start_delta /= psize; > >> > >> If start is not PAGE aligned, then start_delta is not PAGE aligned. > >> so I think the above code will implicitly extend our start to be PAGE aligned. > >> > >> I suggest that we should shrink the start and (start + size) to be PAGE aligned > >> at beginning of this function. > > > > Callers should be with TARGET_PAGE_SIZE aligned on the size, so at least x86_64 > > should be pretty safe since host/guest page sizes match. > > > > Though indeed I must confess I don't know how it worked in general when host > > page size != target page size, at least for migration. For example, I believe > > kvm dirty logging is host page size based, though migration should be migrating > > pages in guest page size granule when it spots a dirty bit set. > > > Hi, > > Indeed, we handle target_page_size aligned @start and @size in general. Maybe we'd better > add explicit function comments about alignment requirement, and explicit alignment assert > on @start and @size? Yes we can, but I think it's not strongly necessary. As Zenghui pointed out, the callers of memory_region_clear_dirty_bitmap() should always be aware of the fact that dirty bitmap is always page size based. OTOH I'm more worried on the other question on how we handle guest psize != host psize case for migration now... Thanks, -- Peter Xu