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To: "Denis V. Lunev" , Konstantin Khorenko Cc: Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, "H . Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Ivanov , Jim Mattson References: <20231109180646.2963718-1-khorenko@virtuozzo.com> <09116ed9-3409-4fbf-9c4f-7a94d8f620aa@virtuozzo.com> <4a0296d4-e4c6-9b90-d805-04284ad1af9f@oracle.com> <12aa9054-73cd-44d3-ba76-f3b59a2bdda3@virtuozzo.com> <12d19ae8-9140-e569-4911-0d8ff8666260@oracle.com> <600ec8cd-bd94-4f82-996f-28225442d5b2@virtuozzo.com> Content-Language: en-US From: Dongli Zhang In-Reply-To: <600ec8cd-bd94-4f82-996f-28225442d5b2@virtuozzo.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-ClientProxiedBy: DS7PR05CA0090.namprd05.prod.outlook.com (2603:10b6:8:56::13) To BYAPR10MB2663.namprd10.prod.outlook.com (2603:10b6:a02:a9::20) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BYAPR10MB2663:EE_|IA1PR10MB7335:EE_ X-MS-Office365-Filtering-Correlation-Id: eb7b9799-2b4c-4a09-5434-08dbe4640534 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 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X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.272,Aquarius:18.0.987,Hydra:6.0.619,FMLib:17.11.176.26 definitions=2023-11-13_06,2023-11-09_01,2023-05-22_02 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 adultscore=0 mlxlogscore=999 spamscore=0 suspectscore=0 phishscore=0 bulkscore=0 malwarescore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2311060000 definitions=main-2311130133 X-Proofpoint-GUID: etOVP7lCHzv1ENA5-sR6yB-SQ1azh0RI X-Proofpoint-ORIG-GUID: etOVP7lCHzv1ENA5-sR6yB-SQ1azh0RI Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/13/23 06:42, Denis V. Lunev wrote: > On 11/13/23 15:14, Dongli Zhang wrote: >> Hi Denis, >> >> On 11/13/23 01:31, Denis V. Lunev wrote: >>> On 11/10/23 01:01, Dongli Zhang wrote: >>>> On 11/9/23 3:46 PM, Denis V. Lunev wrote: >>>>> On 11/9/23 23:52, Jim Mattson wrote: >>>>>> On Thu, Nov 9, 2023 at 10:18 AM Konstantin Khorenko >>>>>> wrote: >>>>>>> Hi All, >>>>>>> >>>>>>> as a followup for my patch: i have noticed that >>>>>>> currently Intel kernel code provides an ability to detect if PMU is totally >>>>>>> disabled for a VM >>>>>>> (pmu->version == 0 in this case), but for AMD code pmu->version is never 0, >>>>>>> no matter if PMU is enabled or disabled for a VM (i mean >>>>>>> in the VM config which >>>>>>> results in "-cpu pmu=off" qemu option). >>>>>>> >>>>>>> So the question is - is it possible to enhance the code for AMD to also >>>>>>> honor >>>>>>> PMU VM setting or it is >>>>>>> impossible by design? >>>>>> The AMD architectural specification prior to AMD PMU v2 does not allow >>>>>> one to describe a CPU (via CPUID or MSRs) that has fewer than 4 >>>>>> general purpose PMU counters. While AMD PMU v2 does allow one to >>>>>> describe such a CPU, legacy software that knows nothing of AMD PMU v2 >>>>>> can expect four counters regardless. >>>>>> >>>>>> Having said that, KVM does provide a per-VM capability for disabling >>>>>> the virtual PMU: KVM_CAP_PMU_CAPABILITY(KVM_PMU_CAP_DISABLE). See >>>>>> section 8.35 in Documentation/virt/kvm/api.rst. >>>>> But this means in particular that QEMU should immediately >>>>> use this KVM_PMU_CAP_DISABLE if this capability is supported and PMU=off. I am >>>>> not seeing this code thus I believe that we have missed this. I think that >>>>> this >>>>> change worth adding. We will measure the impact :-) Den >>>>> >>>> I used to have a patch to use KVM_PMU_CAP_DISABLE in QEMU, but that did not >>>> draw >>>> many developers' attention. >>>> >>>> https://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/20230621013821.6874-2-dongli.zhang@oracle.com/__;!!ACWV5N9M2RV99hQ!McSH2M-kuHmzAwTuXKxrjLkrdJoPqML6cY_Ndc-8k9LRQ7D1V9bSBRQPwHqtx9XCVLK3uzdsMaxyfwve$ >>>> It is time to first re-send that again. >>>> >>>> Dongli Zhang >>> We have checked that setting KVM_PMU_CAP_DISABLE really helps. Konstantin has >>> done this and this is good. On the other hand, looking into these patches I >>> disagree with them. We should not introduce new option for QEMU. If PMU is >>> disabled, i.e. we assume that pmu=off passed in the command line, we should set >>> KVM_PMU_CAP_DISABLE for that virtual machine. Den >> Can I assume you meant pmu=off, that is, cpu->enable_pmu in QEMU? >> >> In my opinion, cpu->enable_pmu indicates the option to control the cpu features. >> It may be used by any accelerators, and it is orthogonal to the KVM cap. >> >> >> The KVM_PMU_CAP_DISABLE is only specific to the KVM accelerator. >> >> >> That's why I had introduced a new option, to allow to configure the VM in my >> dimensions. >> >> It means one dimension to AMD, but two for Intel: to disable PMU via cpuid, or >> KVM cap. >> >> Anyway, this is KVM mailing list, and I may initiate the discussion in QEMU list. >> >> Thank you very much! >> >> Dongli Zhang > with the option pmu='off' it is expected that PMU should be > off for the guest. At the moment (without this KVM capability) > we can disable PMU for Intel only and thus have performance > degradation on AMD. > > This option disables PMU and thus normally when we are > running KVM guest and wanting PMU to be off it would > be required to > * disable CPUID leaf for Intel > * set KVM_PMU_CAP_DISABLE for both processors This would be quite natural and > transparent for the libvirt. Alexander will prepare the patch today or tomorrow > for the discussion. Den That is what I had implemented in the v1 of patch. https://lore.kernel.org/all/20221119122901.2469-3-dongli.zhang@oracle.com/ However, I changed that after people suggested introduce a new property. Dongli Zhang