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Wed, 09 Dec 2020 13:09:09 -0800 (PST) X-Google-Smtp-Source: ABdhPJzKZuGFpl1daSb5ILNFa4veh5w9Fw+x4CFDzimF0yQw1i9otHc7dpVfpAPn9sCbbdJ3arZnYw== X-Received: by 2002:a0c:bf0f:: with SMTP id m15mr5054390qvi.23.1607548149327; Wed, 09 Dec 2020 13:09:09 -0800 (PST) Received: from xz-x1 ([142.126.83.202]) by smtp.gmail.com with ESMTPSA id w15sm1667315qta.95.2020.12.09.13.09.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Dec 2020 13:09:08 -0800 (PST) Date: Wed, 9 Dec 2020 16:09:07 -0500 From: Peter Xu To: Zenghui Yu Subject: Re: [PATCH] kvm: Take into account the unaligned section size when preparing bitmap Message-ID: <20201209210907.GA3211@xz-x1> References: <20201208114013.875-1-yuzenghui@huawei.com> <20201208151654.GA6432@xz-x1> MIME-Version: 1.0 In-Reply-To: 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: 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 Wed, Dec 09, 2020 at 10:33:41AM +0800, Zenghui Yu wrote: > Hi Peter, > > Thanks for having a look at it. > > On 2020/12/8 23:16, Peter Xu wrote: > > Hi, Zenghui, > > > > On Tue, Dec 08, 2020 at 07:40:13PM +0800, Zenghui Yu wrote: > > > The kernel KVM_CLEAR_DIRTY_LOG interface has align requirement on both the > > > start and the size of the given range of pages. We have been careful to > > > handle the unaligned cases when performing CLEAR on one slot. But it seems > > > that we forget to take the unaligned *size* case into account when > > > preparing bitmap for the interface, and we may end up clearing dirty status > > > for pages outside of [start, start + size). > > > > Thanks for the patch, though my understanding is that this is not a bug. > > > > Please have a look at kvm_memslot_init_dirty_bitmap() where we'll allocate the > > dirty bitmap to be aligned to 8 bytes (assuming that's the possible max of the > > value sizeof(unsigned long)). That exactly covers 64 pages. > > > > So here as long as start_delta==0 (so the value of "bmap_npages - size / psize" > > won't really matter a lot, imho), then we'll definitely have KVMSlot.dirty_bmap > > long enough to cover the range we'd like to clear. > > I agree. But actually I'm not saying that KVMSlot.dirty_bmap is not > long enough. What I was having in mind is something like: > > // psize = qemu_real_host_page_size; > // slot.start_addr = 0; > // slot.memory_size = 64 * psize; > > kvm_log_clear_one_slot(slot, as, 0 * psize, 32 * psize); --> [1] > kvm_log_clear_one_slot(slot, as, 32 * psize, 32 * psize); --> [2] > > So the @size is not aligned with 64 pages. Before this patch, we'll > clear dirty status for all pages(0-63) through [1]. It looks to me that > this violates the caller's expectation since we only want to clear > pages(0-31). Now I see; I think you're right. :) > > As I said, I don't think this will happen in practice -- the migration > code should always provide us with a 64-page aligned section (right?). Yes, migration is the major consumer, and that should be guaranteed indeed, see CLEAR_BITMAP_SHIFT_MIN. Not sure about VGA - that should try to do log clear even without migration, but I guess that satisfies the 64-page alignment too, since it's not a huge number (256KB). The VGA buffer size could be related to screen resolution, then N*1024*768 could still guarantee a safe use of the fast path. > I'm just thinking about the correctness of the specific algorithm used > by kvm_log_clear_one_slot(). Yeah, then I think it's okay to have this, just in case someday we'll hit it. Acked-by: Peter Xu (It would be nicer if above example could be squashed into commit message) Thanks, -- Peter Xu