Hi Like, > On Sep 29, 2021, at 5:05 AM, Like Xu <like.xu.linux@xxxxxxxxx> wrote: > > On 29/9/2021 3:35 pm, Peter Zijlstra wrote: >> On Wed, Sep 29, 2021 at 12:04:21AM +0000, Song Liu wrote: >>> Hi Peter, >>> >>> We have see the warning below while testing the new bpf_get_branch_snapshot >>> helper, on a QEMU vm (/usr/bin/qemu-system-x86_64 -enable-kvm -cpu host ...). >>> This issue doesn't happen on bare metal systems (no QEMU). >>> >>> We didn't cover this case, as LBR didn't really work in QEMU. But it seems to >>> work after I upgrade the host kernel to 5.12. > > The guest LBR is enabled since the v5.12. > >>> >>> At the moment, we don't have much idea on how to debug and fix the issue. Could >>> you please share your thoughts on this? >> Well, that's virt, afaik stuff not working is like a feature there or >> something, who knows. I've Cc'ed Like Xu who might have clue since he >> did the patches. >> Virt just ain't worth the pain if you ask me. > > Just cc me for any vPMU/x86 stuff. > >>> >>> Thanks in advance! >>> >>> Song >>> >>> >>> >>> >>> ============================== 8< ============================ >>> >>> [ 139.494159] unchecked MSR access error: WRMSR to 0x3f1 (tried to write 0x0000000000000000) at rIP: 0xffffffff81011a8b (intel_pmu_snapshot_branch_stack+0x3b/0xd0) > > Uh, it uses a PEBS counter to sample or count, which is not yet upstream but should be soon. > > Song, can you try to fix bpf_get_branch_snapshot on a normal PMC counter, > or where is the src for bpf_get_branch_snapshot? I am more than happy to help. bpf_get_branch_snapshot is available in the bpf-next tree: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/ To repro the issue: 1. build the kernel and boot qemu with it; 2. build tools/testing/selftests/bpf cd tools/testing/selftests/bpf ; make -j 3. copy test_progs and bpf_testmod.ko from tools/testing/selftests/bpf to the qemu vm; 4. run the test as ./test_progs -t snapshot -v This should trigger "unchecked MSR access error" in dmesg. Please let me know if this doesn't work. Thanks, Song > >>> [ 139.495587] Call Trace: >>> [ 139.495845] bpf_get_branch_snapshot+0x17/0x40 >>> [ 139.496285] bpf_prog_35810402cd1d294c_test1+0x33/0xe6c >>> [ 139.496791] bpf_trampoline_10737534536_0+0x4c/0x1000 >>> [ 139.497274] bpf_testmod_loop_test+0x5/0x20 [bpf_testmod] >>> [ 139.497799] bpf_testmod_test_read+0x71/0x1f0 [bpf_testmod] >>> [ 139.498332] ? bpf_testmod_loop_test+0x20/0x20 [bpf_testmod] >>> [ 139.498878] ? sysfs_kf_bin_read+0xbe/0x110 >>> [ 139.499284] ? bpf_testmod_loop_test+0x20/0x20 [bpf_testmod] >>> [ 139.499829] kernfs_fop_read_iter+0x1ac/0x2c0 >>> [ 139.500245] ? kernfs_create_link+0x110/0x110 >>> [ 139.500667] new_sync_read+0x24b/0x360 >>> [ 139.501037] ? __x64_sys_llseek+0x1e0/0x1e0 >>> [ 139.501444] ? rcu_read_lock_held_common+0x1a/0x50 >>> [ 139.501942] ? rcu_read_lock_held_common+0x1a/0x50 >>> [ 139.502404] ? rcu_read_lock_sched_held+0x5f/0xd0 >>> [ 139.502865] ? rcu_read_lock_bh_held+0xb0/0xb0 >>> [ 139.503294] ? security_file_permission+0xe7/0x2c0 >>> [ 139.503758] vfs_read+0x1a4/0x2a0 >>> [ 139.504091] ksys_read+0xc0/0x160 >>> [ 139.504413] ? vfs_write+0x510/0x510 >>> [ 139.504756] ? ktime_get_coarse_real_ts64+0xe4/0xf0 >>> [ 139.505234] do_syscall_64+0x3a/0x80 >>> [ 139.505581] entry_SYSCALL_64_after_hwframe+0x44/0xae >>> [ 139.506066] RIP: 0033:0x7fb8a05728b2 >>> [ 139.506413] Code: 97 20 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b6 0f 1f 80 00 00 00 00 f3 0f 1e fa 8b 05 96 db 20 00 85 c0 75 12 31 c0 0f 05 <48> 3d 00 f0 ff ff 77 56 c3 0f 1f 44 00 00 41 54 49 89 d4 55 48 89 >>> [ 139.508164] RSP: 002b:00007ffe66315a28 EFLAGS: 00000246 ORIG_RAX: 0000000000000000 >>> [ 139.508870] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb8a05728b2 >>> [ 139.509545] RDX: 0000000000000064 RSI: 0000000000000000 RDI: 0000000000000010 >>> [ 139.510225] RBP: 00007ffe66315a60 R08: 0000000000000000 R09: 00007ffe66315907 >>> [ 139.510897] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000040c8b0 >>> [ 139.511570] R13: 00007ffe66315cc0 R14: 0000000000000000 R15: 0000000000000000
