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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id o1sm3155256eds.26.2021.02.22.06.18.06 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 22 Feb 2021 06:18:06 -0800 (PST) To: David Hildenbrand , qemu-devel@nongnu.org References: <20210222115708.7623-1-david@redhat.com> <20210222115708.7623-2-david@redhat.com> <7137d1ad-2741-7536-5a3c-58d0c4f8306b@redhat.com> <0277759d-bb9a-6bf3-0ca4-53d3f7ec98f5@redhat.com> From: Paolo Bonzini Subject: Re: [PATCH v6 01/12] memory: Introduce RamDiscardMgr for RAM memory regions Message-ID: Date: Mon, 22 Feb 2021 15:18:05 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0 MIME-Version: 1.0 In-Reply-To: <0277759d-bb9a-6bf3-0ca4-53d3f7ec98f5@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=pbonzini@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 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, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=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: Pankaj Gupta , Wei Yang , "Michael S . Tsirkin" , Alex Williamson , Peter Xu , "Dr . David Alan Gilbert" , Auger Eric , Pankaj Gupta , teawater , Igor Mammedov , Marek Kedzierski Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 22/02/21 15:03, David Hildenbrand wrote: > >>> +    /** >>> +     * @replay_populated: >>> +     * >>> +     * Notify the #RamDiscardListener about all populated parts >>> within the >>> +     * #MemoryRegion via the #RamDiscardMgr. >>> +     * >>> +     * In case any notification fails, no further notifications are >>> triggered. >>> +     * The listener is not required to be registered. >>> +     * >>> +     * @rdm: the #RamDiscardMgr >>> +     * @mr: the #MemoryRegion >>> +     * @rdl: the #RamDiscardListener >>> +     * >>> +     * Returns 0 on success, or a negative error if any notification >>> failed. >>> +     */ >>> +    int (*replay_populated)(const RamDiscardMgr *rdm, const >>> MemoryRegion *mr, >>> +                            RamDiscardListener *rdl); >> >> If this function is only going to use a single callback, just pass it >> (together with a void *opaque) as the argument. >>> +}; >>> + >>>   typedef struct CoalescedMemoryRange CoalescedMemoryRange; >>>   typedef struct MemoryRegionIoeventfd MemoryRegionIoeventfd; >>> @@ -487,6 +683,7 @@ struct MemoryRegion { >>>       const char *name; >>>       unsigned ioeventfd_nb; >>>       MemoryRegionIoeventfd *ioeventfds; >>> +    RamDiscardMgr *rdm; /* Only for RAM */ >>>   }; >> >> >> The idea of sending discard notifications is obviously good.  I have a >> couple of questions on the design that you used for the interface; I'm >> not saying that it should be done differently, I would only like to >> understand the trade offs that you chose: > > Sure! > >> >> 1) can the RamDiscardManager (no abbreviations :)) be just the owner of > > I used to call it "SparseRamManager", but wanted to stress the semantics > - and can use RamDiscardManager ;) . Suggestions welcome. > >> the memory region (obj->parent)?  Alternatively, if you want to make it >> separate from the owner, does it make sense for it to be a separate >> reusable object, sitting between virtio-mem and the MemoryRegion, so >> that the implementation can be reused? > > virtio-mem consumes a memory backend (e.g., memory-backend-ram). That > one logically "ownes" the memory region (and thereby the RAMBlock). > > The memory backend gets assigned to virtio-mem. At that point, > virtio-mem "owns" the memory backend. It will set itself as the > RamDiscardsManager before mapping the memory region into system address > space (whereby it will get exposed to the system). > > This flow made sense to me. Regarding "reusable object" - I think the > only stuff we could fit in there would be e.g., maintaining the lists of > notifiers. I'd rather wait until we actually have a second user to see > what we can factor out. > > If you have any suggestion/preference, please let me know. > >> >> 2) why have the new RamDiscardListener instead of just extending >> MemoryListener? Conveniently, vfio already has a MemoryListener that can > > It behaves more like the IOMMU notifier in that regard. Yes, but does it behave more like the IOMMU notifier in other regards? :) The IOMMU notifier is concerned with an iova concept that doesn't exist at the MemoryRegion level, while RamDiscardListener works at the (MemoryRegion, offset) level that can easily be represented by a MemoryRegionSection. Using MemoryRegionSection might even simplify the listener code. >> be extended, and you wouldn't need the list of RamDiscardListeners. >> There is already a precedent of replaying the current state when a >> listener is added (see listener_add_address_space), so this is not >> something different between ML and RDL. > > The main motivation is to let listener decide how it wants to handle the > memory region. For example, for vhost, vdpa, kvm, ... I only want a > single region, not separate ones for each and every populated range, > punching out discarded ranges. Note that there are cases (i.e., > anonymous memory), where it's even valid for the guest to read discarded > memory. Yes, I agree with that. You would still have the same region-add/region_nop/region_del callbacks for KVM and friends; on top of that you would have region_populate/region_discard callbacks for VFIO. Populated regions would be replayed after region_add, while I don't think it makes sense to have a region_discard_all callback before region_discard. Paolo > Special cases are only required in corner cases, namely whenever we > unconditionally: > > a) Read memory inside a memory region. (e.g., guest-memory-dump) > b) Write memory inside a memory region. (e.g., TPM, migration) > c) Populate memory inside a memory region. (e.g., vfio) > >> >> Also, if you add a new interface, you should have "method call" wrappers >> similar to user_creatable_complete or user_creatable_can_be_deleted. > > I think I had those at some point but decided to drop them. Can readd them. > >