On Fri, Aug 11, 2017 at 12:21:16AM +0300, Michael S. Tsirkin wrote: > On Thu, Aug 10, 2017 at 05:40:34PM +0100, Richard W.M. Jones wrote: > > If using indirect descriptors, you can make the total_sg as large as > > you want. > > That would be a spec violation though, even if it happens to > work on current QEMU. > > The spec says: > A driver MUST NOT create a descriptor chain longer than the Queue Size of the device. > > What prompted this patch? > Do we ever encounter this situation? This patch is needed because the following (2/2) patch will trigger that BUG_ON if I set virtqueue_size=64 or any smaller value. The precise backtrace is below. Rich. [ 4.029510] ------------[ cut here ]------------ [ 4.030127] kernel BUG at drivers/virtio/virtio_ring.c:299! [ 4.030834] invalid opcode: 0000 [#1] SMP [ 4.031340] Modules linked in: libcrc32c crc8 crc7 crc4 crc_itu_t virtio_pci virtio_mmio virtio_input virtio_balloon virtio_scsi nd_pmem nd_btt virtio_net virtio_crypto crypto_engine virtio_console virtio_rng virtio_blk virtio_ring virtio nfit crc32_generic crct10dif_pclmul crc32c_intel crc32_pclmul [ 4.034606] CPU: 0 PID: 1 Comm: init Not tainted 4.13.0-rc4+ #100 [ 4.035354] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014 [ 4.036770] task: ffff9a859e243300 task.stack: ffffa731c00cc000 [ 4.037506] RIP: 0010:virtqueue_add_sgs+0x23d/0x460 [virtio_ring] [ 4.038250] RSP: 0000:ffffa731c00cf6e0 EFLAGS: 00010097 [ 4.038898] RAX: 0000000000000000 RBX: 0000000000000011 RCX: ffffdd0680646c40 [ 4.039762] RDX: 0000000000000000 RSI: ffffa731c00cf7b8 RDI: ffff9a85945c4d68 [ 4.040634] RBP: ffffa731c00cf748 R08: ffff9a85945c4a78 R09: 0000000001080020 [ 4.041508] R10: ffffa731c00cf788 R11: ffff9a859b3d3120 R12: ffffa731c00cf7d0 [ 4.042382] R13: ffffa731c00cf7d0 R14: 0000000000000001 R15: ffff9a859b3f8200 [ 4.043248] FS: 0000000000000000(0000) GS:ffff9a859e600000(0000) knlGS:0000000000000000 [ 4.044232] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 4.044942] CR2: 00007fcff02e931c CR3: 000000001d23b000 CR4: 00000000003406f0 [ 4.045815] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 [ 4.046684] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 [ 4.047559] Call Trace: [ 4.047876] virtscsi_add_cmd+0x1c9/0x280 [virtio_scsi] [ 4.048528] virtscsi_kick_cmd+0x38/0x90 [virtio_scsi] [ 4.049161] virtscsi_queuecommand+0x104/0x280 [virtio_scsi] [ 4.049875] virtscsi_queuecommand_single+0x38/0x40 [virtio_scsi] [ 4.050628] scsi_dispatch_cmd+0xf9/0x390 [ 4.051128] scsi_queue_rq+0x5e5/0x6f0 [ 4.051602] blk_mq_dispatch_rq_list+0x1ff/0x3c0 [ 4.052175] blk_mq_sched_dispatch_requests+0x199/0x1f0 [ 4.052824] __blk_mq_run_hw_queue+0x11d/0x1b0 [ 4.053382] __blk_mq_delay_run_hw_queue+0x8d/0xa0 [ 4.053972] blk_mq_run_hw_queue+0x14/0x20 [ 4.054485] blk_mq_sched_insert_requests+0x96/0x120 [ 4.055095] blk_mq_flush_plug_list+0x19d/0x410 [ 4.055661] blk_flush_plug_list+0xf9/0x2b0 [ 4.056182] blk_finish_plug+0x2c/0x40 [ 4.056655] __do_page_cache_readahead+0x32e/0x400 [ 4.057261] filemap_fault+0x2fb/0x890 [ 4.057731] ? filemap_fault+0x2fb/0x890 [ 4.058220] ? find_held_lock+0x3c/0xb0 [ 4.058714] ext4_filemap_fault+0x34/0x50 [ 4.059212] __do_fault+0x1e/0x110 [ 4.059644] __handle_mm_fault+0x6b2/0x1080 [ 4.060167] handle_mm_fault+0x178/0x350 [ 4.060662] __do_page_fault+0x26e/0x510 [ 4.061152] trace_do_page_fault+0x9d/0x290 [ 4.061677] do_async_page_fault+0x51/0xa0 [ 4.062189] async_page_fault+0x28/0x30 [ 4.062667] RIP: 0033:0x7fcff030a24f [ 4.063113] RSP: 002b:00007ffefc2ad078 EFLAGS: 00010206 [ 4.063760] RAX: 00007fcff02e931c RBX: 00007fcff050f660 RCX: 00007fcff02e935c [ 4.064648] RDX: 0000000000000664 RSI: 0000000000000000 RDI: 00007fcff02e931c [ 4.065519] RBP: 00007ffefc2ad320 R08: 00007fcff02e931c R09: 0000000000027000 [ 4.066392] R10: 00007fcff02e9980 R11: 0000000000000206 R12: 00007ffefc2ad0b0 [ 4.067263] R13: 00007ffefc2ad408 R14: 0000000000000002 R15: 00000000000087f0 [ 4.068135] Code: af 01 c7 45 c8 01 00 00 00 45 31 ed e9 9b fe ff ff 31 db 48 83 7d d0 00 0f 85 3f fe ff ff 0f 0b 48 8b 7d b8 e8 e5 fd 22 de eb 8b <0f> 0b 0f 0b 44 89 6d a8 45 89 f5 48 8b 45 a0 44 8d 70 01 48 83 [ 4.070506] RIP: virtqueue_add_sgs+0x23d/0x460 [virtio_ring] RSP: ffffa731c00cf6e0 [ 4.071434] ---[ end trace 02532659840e2a64 ]--- -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com libguestfs lets you edit virtual machines. Supports shell scripting, bindings from many languages. http://libguestfs.org
