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Wed, 29 Sep 2021 14:42:27 +0000 From: Song Liu To: Peter Zijlstra CC: Like Xu , bpf , Networking , Kernel Team , Andrii Nakryiko , Alexei Starovoitov , "like.xu@linux.intel.com" Subject: Re: bpf_get_branch_snapshot on qemu-kvm Thread-Topic: bpf_get_branch_snapshot on qemu-kvm Thread-Index: AQHXtMWNyKWclBi4s0iJww6HNTXDJ6u6n6yAgABLWQCAAAXKgIAAJheA Date: Wed, 29 Sep 2021 14:42:27 +0000 Message-ID: References: <0E5E6FCA-23ED-4CAA-ADEA-967430C62F6F@fb.com> In-Reply-To: Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-mailer: Apple Mail (2.3654.120.0.1.13) authentication-results: infradead.org; dkim=none (message not signed) header.d=none;infradead.org; dmarc=none action=none header.from=fb.com; x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: bd153938-06a9-460c-b15f-08d983575bdd x-ms-traffictypediagnostic: SA1PR15MB5061: x-ms-exchange-transport-forked: True x-microsoft-antispam-prvs: x-fb-source: Internal x-ms-oob-tlc-oobclassifiers: OLM:7219; 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charset="us-ascii" Content-ID: <789EDB1D1EFBB24B9A191DA057050A44@namprd15.prod.outlook.com> MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: SA1PR15MB5109.namprd15.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: bd153938-06a9-460c-b15f-08d983575bdd X-MS-Exchange-CrossTenant-originalarrivaltime: 29 Sep 2021 14:42:27.5816 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: O5bHcQxcZ48k4y4E1lpP8Bzc1rI9XlrVo7s3ZN9BpVZaoUJgUHjoAMuAorEAbMkwoj0GE0Op6yIaH1einiBK/g== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR15MB5061 X-OriginatorOrg: fb.com X-Proofpoint-ORIG-GUID: yGQznNW0S90O9Ilhpx6qcsjhKDItNTku X-Proofpoint-GUID: yGQznNW0S90O9Ilhpx6qcsjhKDItNTku X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475 definitions=2021-09-29_06,2021-09-29_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 bulkscore=0 priorityscore=1501 spamscore=0 clxscore=1015 suspectscore=0 mlxlogscore=823 mlxscore=0 adultscore=0 malwarescore=0 phishscore=0 lowpriorityscore=0 impostorscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109230001 definitions=main-2109290090 X-FB-Internal: deliver Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org Hi Peter, > On Sep 29, 2021, at 5:26 AM, Peter Zijlstra wrote: > > On Wed, Sep 29, 2021 at 08:05:24PM +0800, Like Xu wrote: >> On 29/9/2021 3:35 pm, Peter Zijlstra wrote: > >>>> [ 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. > > Ooh that's PEBS_ENABLE > >> 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. > > Nah, all that code wants to do is disable PEBS... and virt being virt, > it's all sorts of weird with f/m/s :/ > > I so hate all that. So there's two solutions: > > - get confirmation that clearing GLOBAL_CTRL is suffient to supress > PEBS, in which case we can simply remove the PEBS_ENABLE clear. How should we confirm this? Can we run some tests for this? Or do we need hardware experts' input for this? Thanks, Song > > or > > - create another 2 intel_pmu_snapshot_{,arch_}branch_stack() copies > that omit the PEBS thing. >