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David Alan Gilbert" , Markus Armbruster , Peter Xu References: <20210318174611.293520-1-andrey.gruzdev@virtuozzo.com> <20210318174611.293520-4-andrey.gruzdev@virtuozzo.com> From: Andrey Gruzdev Message-ID: Date: Fri, 19 Mar 2021 14:05:47 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Originating-IP: [95.165.26.68] X-ClientProxiedBy: AM4PR0501CA0064.eurprd05.prod.outlook.com (2603:10a6:200:68::32) To AM0PR08MB3364.eurprd08.prod.outlook.com (2603:10a6:208:e4::15) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [192.168.1.64] (95.165.26.68) by AM4PR0501CA0064.eurprd05.prod.outlook.com (2603:10a6:200:68::32) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3955.18 via Frontend Transport; Fri, 19 Mar 2021 11:05:48 +0000 X-MS-PublicTrafficType: Email X-MS-Office365-Filtering-Correlation-Id: 46f8ea3d-970b-41a1-3236-08d8eac6f3d7 X-MS-TrafficTypeDiagnostic: AM0PR08MB5410: X-MS-Exchange-Transport-Forked: True X-Microsoft-Antispam-PRVS: X-MS-Oob-TLC-OOBClassifiers: OLM:9508; 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envelope-from=andrey.gruzdev@virtuozzo.com; helo=EUR05-VI1-obe.outbound.protection.outlook.com X-Spam_score_int: -26 X-Spam_score: -2.7 X-Spam_bar: -- X-Spam_report: (-2.7 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, RCVD_IN_SBL=0.141, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=no 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 19.03.2021 12:28, David Hildenbrand wrote: >> +/* >> + * ram_block_populate_pages: populate memory in the RAM block by >> reading >> + *   an integer from the beginning of each page. >> + * >> + * Since it's solely used for userfault_fd WP feature, here we just >> + *   hardcode page size to TARGET_PAGE_SIZE. >> + * >> + * @bs: RAM block to populate >> + */ >> +volatile int ram_block_populate_pages__tmp; >> +static void ram_block_populate_pages(RAMBlock *bs) >> +{ >> +    ram_addr_t offset = 0; >> +    int tmp = 0; >> + >> +    for (char *ptr = (char *) bs->host; offset < bs->used_length; >> +            ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) { > > You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE > Ok. >> +        /* Try to do it without memory writes */ >> +        tmp += *(volatile int *) ptr; >> +    } > > > The following is slightly simpler and doesn't rely on volatile > semantics [1]. > Should work on any arch I guess. > > static void ram_block_populate_pages(RAMBlock *bs) > { >     char *ptr = (char *) bs->host; >     ram_addr_t offset; > >     for (offset = 0; offset < bs->used_length; >          offset += qemu_real_host_page_size) { >     char tmp = *(volatile char *)(ptr + offset) > >     /* Don't optimize the read out. */ >     asm volatile ("" : "+r" (tmp)); > } > Thanks, good option, I'll change the code. > Compiles to > >     for (offset = 0; offset < bs->used_length; >     316d:       48 8b 4b 30             mov    0x30(%rbx),%rcx >     char *ptr = (char *) bs->host; >     3171:       48 8b 73 18             mov    0x18(%rbx),%rsi >     for (offset = 0; offset < bs->used_length; >     3175:       48 85 c9                test   %rcx,%rcx >     3178:       74 ce                   je     3148 > >          offset += qemu_real_host_page_size) { >     317a:       48 8b 05 00 00 00 00    mov 0x0(%rip),%rax        # > 3181 >     3181:       48 8b 38                mov    (%rax),%rdi >     3184:       0f 1f 40 00             nopl   0x0(%rax) >         char tmp = *(volatile char *)(ptr + offset); >     3188:       48 8d 04 16             lea    (%rsi,%rdx,1),%rax >     318c:       0f b6 00                movzbl (%rax),%eax >          offset += qemu_real_host_page_size) { >     318f:       48 01 fa                add    %rdi,%rdx >     for (offset = 0; offset < bs->used_length; >     3192:       48 39 ca                cmp    %rcx,%rdx >     3195:       72 f1                   jb     3188 > > > > [1] > https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/ > > > I'll send patches soon to take care of virtio-mem via RamDiscardManager - > to skip populating the parts that are supposed to remain discarded and > not migrated. > Unfortunately, the RamDiscardManager patches are still stuck waiting for > acks ... and now we're in soft-freeze. > RamDiscardManager patches - do they also modify migration code? I mean which part is responsible of not migrating discarded ranges. -- Andrey Gruzdev, Principal Engineer Virtuozzo GmbH +7-903-247-6397 virtuzzo.com