Add KNOWN_BUGS file

Document known major (e.g. linux crash) bugs that have not been
fixed downstream and might require workarounds on specific
hardware configurations

Change-Id: I51e5d23243afd4489ce1ae25e736afc27b2c8202

KNOWN_BUGS.md

@@ -0,0 +1,70 @@
+Linux crash when offlining CPU (el7, hardware-specific)
+=========================================================
+
+On some hardware with el7 kernel, linux can crash due to a bug in the
+irq handling when offlining CPUs (reserve cpu part of mcreboot)
+
+Example stack trace:
+```
+[ 4147.052753] BUG: unable to handle kernel NULL pointer dereference at 0000000000000040
+[ 4147.060677] IP: [<ffffffff8102ce26>] check_irq_vectors_for_cpu_disable+0x86/0x1c0
+[ 4147.068226] PGD 1057e44067 PUD 105f1e7067 PMD 0
+[ 4147.072935] Oops: 0000 [#1] SMP
+[ 4147.076230] Modules linked in: mcctrl(OE) ihk_smp_x86_64(OE) ihk(OE) xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun rpcsec_gss_krb5 nfsv4 dns_resolver nfs fscache ip6t_rpfilter ipt_REJECT nf_reject_ipv4 ip6t_REJECT nf_reject_ipv6 xt_conntrack ip_set nfnetlink ebtable_nat ebtable_broute bridge stp llc ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter rpcrdma ib_isert iscsi_target_mod ib_iser libiscsi scsi_transport_iscsi ib_srpt target_core_mod ib_srp scsi_transport_srp scsi_tgt ib_ipoib rdma_ucm ib_ucm ib_uverbs ib_umad rdma_cm ib_cm iw_cm mlx4_ib ib_core
+[ 4147.148619]  dm_mirror dm_region_hash dm_log dm_mod sb_edac edac_core intel_powerclamp coretemp ext4 mbcache jbd2 intel_rapl iosf_mbi kvm_intel kvm irqbypass crc32_pclmul ghash_clmulni_intel aesni_intel lrw gf128mul ipmi_ssif glue_helper ablk_helper joydev iTCO_wdt iTCO_vendor_support cryptd ipmi_si ipmi_devintf ipmi_msghandler pcspkr wmi mei_me mei lpc_ich i2c_i801 sg ioatdma shpchp nfsd auth_rpcgss nfs_acl lockd grace sunrpc ip_tables xfs libcrc32c mlx4_en sd_mod crc_t10dif crct10dif_generic mgag200 drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm isci igb drm mlx4_core libsas ahci libahci scsi_transport_sas libata crct10dif_pclmul ptp crct10dif_common pps_core crc32c_intel dca i2c_algo_bit i2c_core devlink [last unloaded: ihk]
+[ 4147.215370] CPU: 6 PID: 38 Comm: migration/6 Tainted: G           OE  ------------ T 3.10.0-693.2.2.el7.x86_64 #1
+[ 4147.225672] Hardware name: SGI.COM C1104G-RP5/X9DRG-HF, BIOS 3.0  10/25/2013
+[ 4147.232747] task: ffff880174689fa0 ti: ffff8801746ac000 task.ti: ffff8801746ac000
+[ 4147.240278] RIP: 0010:[<ffffffff8102ce26>]  [<ffffffff8102ce26>] check_irq_vectors_for_cpu_disable+0x86/0x1c0
+[ 4147.250275] RSP: 0018:ffff8801746afd30  EFLAGS: 00010046
+[ 4147.255608] RAX: 0000000000000000 RBX: 000000000000004e RCX: 0000000000000000
+[ 4147.262770] RDX: 0000000000000020 RSI: 000000000000005f RDI: 0000000000000023
+[ 4147.269936] RBP: ffff8801746afd58 R08: 0000000000000001 R09: ffff88017f800490
+[ 4147.277103] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000006
+[ 4147.284269] R13: 0000000000000000 R14: ffff88085ca82500 R15: 000000000000005f
+[ 4147.291429] FS:  0000000000000000(0000) GS:ffff88085fb80000(0000) knlGS:0000000000000000
+[ 4147.299556] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
+[ 4147.305326] CR2: 0000000000000040 CR3: 0000001059704000 CR4: 00000000001407e0
+[ 4147.312490] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
+[ 4147.319659] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
+[ 4147.326827] Stack:
+[ 4147.328857]  ffff8808f43078c8 ffff8808f4307850 0000000000000286 ffff8808f4307701
+[ 4147.336384]  0000000000000000 ffff8801746afd70 ffffffff81052a82 0000000200000000
+[ 4147.343915]  ffff8801746afd88 ffffffff81693ca3 0000000000000003 ffff8801746afdc0
+[ 4147.351447] Call Trace:
+[ 4147.353921]  [<ffffffff81052a82>] native_cpu_disable+0x12/0x40
+[ 4147.359795]  [<ffffffff81693ca3>] take_cpu_down+0x13/0x40
+[ 4147.365236]  [<ffffffff81116899>] multi_cpu_stop+0xd9/0x100
+[ 4147.370850]  [<ffffffff811167c0>] ? cpu_stop_should_run+0x50/0x50
+[ 4147.376983]  [<ffffffff81116ab7>] cpu_stopper_thread+0x97/0x150
+[ 4147.382942]  [<ffffffff816a8fad>] ? __schedule+0x39d/0x8b0
+[ 4147.388461]  [<ffffffff810b909f>] smpboot_thread_fn+0x12f/0x180
+[ 4147.394406]  [<ffffffff810b8f70>] ? lg_double_unlock+0x40/0x40
+[ 4147.400276]  [<ffffffff810b098f>] kthread+0xcf/0xe0
+[ 4147.405182]  [<ffffffff810b08c0>] ? insert_kthread_work+0x40/0x40
+[ 4147.411319]  [<ffffffff816b4f58>] ret_from_fork+0x58/0x90
+[ 4147.418893]  [<ffffffff810b08c0>] ? insert_kthread_work+0x40/0x40
+[ 4147.426524] Code: 81 fb 00 01 00 00 0f 84 8a 00 00 00 89 d8 65 44 8b 3c 85 20 c6 00 00 45 85 ff 78 e1 44 89 ff e8 91 31 10 00 48 63 15 7e 10 af 00 <48> 8b 70 40 48 c7 c7 80 71 cf 81 49 89 c6 48 83 c2 3f 48 c1 fa
+[ 4147.450352] RIP  [<ffffffff8102ce26>] check_irq_vectors_for_cpu_disable+0x86/0x1c0
+[ 4147.460135]  RSP <ffff8801746afd30>
+[ 4147.465154] CR2: 0000000000000040
+```
+
+This bug has been fixed upstream, but redhat will not backport the fixes.
+You can work around the problem with a kpatch by backporting the three
+following commits:
+
+x86: irq: Get correct available vectors for cpu disable
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ac2a55395eddccd6e3e39532df9869d61e97b2ee
+
+x86/irq: Check for valid irq descriptor in check_irq_vectors_for_cpu_disable()
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d97eb8966c91f2c9d05f0a22eb89ed5b76d966d1
+
+x86/irq: Use proper locking in check_irq_vectors_for_cpu_disable()
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cbb24dc761d95fe39a7a122bb1b298e9604cae15
+
+
+Alternatively, since it is related to the irq configuration, it might
+be possible to mitigate the issue by setting the irq affinities early
+on and making sure none of the cpus that will be offlined have any irq
+configured.