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Darwish" , Alexandre Chartre , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out Message-ID: References: <20251020194446.w2s5f6nr7i7vw4ve@desk> <20251020204026.a7xuhnagsqo4otpx@desk> <225134fd-033f-4d63-b88c-772179054694@intel.com> <20251020224159.xkhfs3phai5o6rzb@desk> <1D952EBC-CA16-49FE-8AD0-48BCE038332B@nutanix.com> <73a22a19-c492-4a75-8352-a4cfac47d812@intel.com> <883EDC14-1A26-46F4-B9E9-C75A6DF07195@nutanix.com> <5E09F6D2-F6E5-45A2-8264-34DC6DF679B5@nutanix.com> In-Reply-To: <5E09F6D2-F6E5-45A2-8264-34DC6DF679B5@nutanix.com> X-ClientProxiedBy: BL6PEPF00016418.NAMP222.PROD.OUTLOOK.COM (2603:10b6:22e:400:0:1004:0:7) To DM4PR10MB8218.namprd10.prod.outlook.com (2603:10b6:8:1cc::16) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR10MB8218:EE_|IA4PR10MB8400:EE_ X-MS-Office365-Filtering-Correlation-Id: 22facb8c-78d1-4ae7-6bad-08de10b90e4f X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|7416014|376014|1800799024; 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charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUxMDE4MDAyMyBTYWx0ZWRfXx2lLJ/iZOdxL wByoqMb9lpGRp/9+tUFyu9rkYqf0zGPxuaoOyg1+BeilTxbOYsn5pj1hNWwv0frvUVL6P2iiIUp vDnSK2PIpJ/qUdFiHCgWzjQ6cH5ei9LVS7oRUFdl7T4j7PEQ8gQtbrBbPhQ93s4eroGhDAmSYkZ efEgwXrLx0GHF9IZqCKNvbMEsS9dhRWdclPMHfdLvPA+ztsYUWIazDN4ylS/rJblhV+EGKHrROY 7SJ3WdCTnKAzn8ghdATL9j6i5SO6ngCYoRBxbvymGYCyyqQLZ6TDAEhN13GjCljR67rEZr+Uvox t7h0OlYHWa+zLE65qHkZFOZLIpR9YXy83rUwU6vzOY6D2dgAa/L3txzeaG9t7dQk4sLVClb3xSr SPXgT6Q6LP/mab5qeqqCH7qijKqNXg== X-Proofpoint-GUID: KShn7oyNodkS4Eo1WbS1X2_pVf0GyrDY X-Authority-Analysis: v=2.4 cv=KoZAGGWN c=1 sm=1 tr=0 ts=68f7aaf4 b=1 cx=c_pps a=WeWmnZmh0fydH62SvGsd2A==:117 a=WeWmnZmh0fydH62SvGsd2A==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=x6icFKpwvdMA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=yPCof4ZbAAAA:8 a=QyXUC8HyAAAA:8 a=gs9Pb4jnFpyxObAF27wA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: KShn7oyNodkS4Eo1WbS1X2_pVf0GyrDY On Tue, Oct 21, 2025 at 03:35:28PM +0000, Jon Kohler wrote: >=20 >=20 > > On Oct 21, 2025, at 11:27=E2=80=AFAM, Konrad Rzeszutek Wilk wrote: > >=20 > > > >=20 > > On Tue, Oct 21, 2025 at 02:39:15PM +0000, Jon Kohler wrote: > >>=20 > >>=20 > >>> On Oct 21, 2025, at 10:01=E2=80=AFAM, Dave Hansen wrote: > >>>=20 > >>>>>=20 > >>> On 10/21/25 06:40, Jon Kohler wrote: > >>>> So to simplify it down: > >>>> A guest VM that updates to a ITS-enabled guest kernel sees performan= ce > >>>> impacts on non-vulnerable hardware, when running on non-BHI_CTRL and= /or > >>>> non-ITS_NO hypervisors, which is a very easy situation to get into, = especially > >>>> on QEMU with live migration-enabled pools. > >>>=20 > >>> By non-$FEATURE, do you mean that they chose to not enumerate those > >>> features, or that they are completely ignorant of them? > >>=20 > >> Both cases are true for QEMU. > >>=20 > >> For ITS_NO, it is an allowed feature, but its not part of a QEMU model= by > >> default, so the higher level control plane whatever that may be would = need to > >> specifically turn it on, its not automatic. > >>=20 > >> For BHI_CTRL, depending on what QEMU the VM was originally *started* o= n, > >> the guest may have access to Sapphire Rapids models, but BHI_CTRL may > >> not have existed in the QEMU source at that time, as those were introd= uced > >> into two different timeframes. > >=20 > > QEMU provides now a mechanism to update itself to a newer version. See > >=20 > > https://urldefense.proofpoint.com/v2/url?u=3Dhttps-3A__blogs.oracle.com= _linux_post_qemu-2Dlive-2Dupdate&d=3DDwIFaQ&c=3Ds883GpUCOChKOHiocYtGcg&r=3D= NGPRGGo37mQiSXgHKm5rCQ&m=3DUUSvpPViiTB6CJQtj3GREK4bQFz7MT9uNiAu5AL3O23d6I1y= k4vheLDyR41ZcbRI&s=3Dx19gwHU3HGXSlGkK0ppkawex3SIbs8xHj5hPtwNCFwc&e=3D=20 > >=20 > > That should solve your QEMU problem. >=20 > Can this live update feature change CPU feature bits (e.g. add on > -cpu ModelHere =E2=80=A6 new_flag=3Dyes) during the update? It is possible, but a pain in operation b/c you have to keep track of the newer parameters you are adding and if add some device in wrong order (wrong PCI BDF), well, things won't be nice. >=20 > Can it change CPU models during the live update (e.g. change > -cpu Model-v1 to -cpu Model-v2)? Not exactly. But you can modify the built-in Model-v1 (in the newer version) to have the newer CPU flags and it will carry-over.=20 Anyhow it is all in the latest version of QEMU.