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[91.12.99.11]) by smtp.gmail.com with ESMTPSA id m32sm5632747wms.2.2021.08.05.01.20.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Aug 2021 01:20:59 -0700 (PDT) Subject: Re: [PATCH v3 6/7] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination To: =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-devel@nongnu.org References: <20210730085249.8246-1-david@redhat.com> <20210730085249.8246-7-david@redhat.com> <5f8c6173-046d-9fc2-c649-93ede45ca77d@redhat.com> <2c8d80ad-f171-7d5f-3235-92f02fa174b3@redhat.com> From: David Hildenbrand Organization: Red Hat Message-ID: Date: Thu, 5 Aug 2021 10:20:58 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <2c8d80ad-f171-7d5f-3235-92f02fa174b3@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=david@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=170.10.133.124; envelope-from=david@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.699, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.132, RCVD_IN_DNSWL_LOW=-0.7, 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: Alex Williamson , Eduardo Habkost , Juan Quintela , "Michael S. Tsirkin" , "Dr. David Alan Gilbert" , Peter Xu , Andrey Gruzdev , Pankaj Gupta , teawater , Paolo Bonzini , Marek Kedzierski , Wei Yang Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 05.08.21 10:17, Philippe Mathieu-Daudé wrote: > On 8/5/21 10:07 AM, David Hildenbrand wrote: >> On 05.08.21 09:48, Philippe Mathieu-Daudé wrote: >>> On 7/30/21 10:52 AM, David Hildenbrand wrote: >>>> Currently, when someone (i.e., the VM) accesses discarded parts inside a >>>> RAMBlock with a RamDiscardManager managing the corresponding mapped >>>> memory >>>> region, postcopy will request migration of the corresponding page >>>> from the >>>> source. The source, however, will never answer, because it refuses to >>>> migrate such pages with undefined content ("logically unplugged"): the >>>> pages are never dirty, and get_queued_page() will consequently skip >>>> processing these postcopy requests. >>>> >>>> Especially reading discarded ("logically unplugged") ranges is >>>> supposed to >>>> work in some setups (for example with current virtio-mem), although it >>>> barely ever happens: still, not placing a page would currently stall the >>>> VM, as it cannot make forward progress. >>>> >>>> Let's check the state via the RamDiscardManager (the state e.g., >>>> of virtio-mem is migrated during precopy) and avoid sending a request >>>> that will never get answered. Place a fresh zero page instead to keep >>>> the VM working. This is the same behavior that would happen >>>> automatically without userfaultfd being active, when accessing virtual >>>> memory regions without populated pages -- "populate on demand". >>>> >>>> For now, there are valid cases (as documented in the virtio-mem spec) >>>> where >>>> a VM might read discarded memory; in the future, we will disallow that. >>>> Then, we might want to handle that case differently, e.g., warning the >>>> user that the VM seems to be mis-behaving. >>>> >>>> Signed-off-by: David Hildenbrand >>>> --- >>>>   migration/postcopy-ram.c | 31 +++++++++++++++++++++++++++---- >>>>   migration/ram.c          | 21 +++++++++++++++++++++ >>>>   migration/ram.h          |  1 + >>>>   3 files changed, 49 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c >>>> index 2e9697bdd2..38cdfc09c3 100644 >>>> --- a/migration/postcopy-ram.c >>>> +++ b/migration/postcopy-ram.c >>>> @@ -671,6 +671,29 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, >>>>       return ret; >>>>   } >>>>   +static int postcopy_request_page(MigrationIncomingState *mis, >>>> RAMBlock *rb, >>>> +                                 ram_addr_t start, uint64_t haddr) >>>> +{ >>>> +    void *aligned = (void *)(uintptr_t)(haddr & >>>> -qemu_ram_pagesize(rb)); >>> >>>    void *aligned = QEMU_ALIGN_PTR_DOWN(haddr, qemu_ram_pagesize(rb))); >>> >> >> Does not compile as haddr is not a pointer. > > I suppose the typeof() fails? > > /* n-byte align pointer down */ > #define QEMU_ALIGN_PTR_DOWN(p, n) \ > ((typeof(p))QEMU_ALIGN_DOWN((uintptr_t)(p), (n))) > > >> void *aligned = QEMU_ALIGN_PTR_DOWN((void *)haddr, qemu_ram_pagesize(rb))); >> >> should work. > > What about > > void *aligned = QEMU_ALIGN_DOWN(haddr, qemu_ram_pagesize(rb))); > > else > > void *aligned = (void *)QEMU_ALIGN_DOWN(haddr, qemu_ram_pagesize(rb))); That works as well and will use that for now. At one point we should just pass a pointer instead of uint64_t for the host address. > > I don't mind much the style you prefer, as long as it compiles :p > But these QEMU_ALIGN_DOWN() macros make the review easier than (a & -b). > Yes, absolutely. I'll add a patch converting a bunch of such alignments in migration code. -- Thanks, David / dhildenb