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mx.microsoft.com 1; spf=pass smtp.mailfrom=intel.com; dmarc=pass action=none header.from=intel.com; dkim=pass header.d=intel.com; arc=none Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=intel.com; Received: from DS0PR11MB7529.namprd11.prod.outlook.com (2603:10b6:8:141::20) by DM6PR11MB4689.namprd11.prod.outlook.com (2603:10b6:5:2a0::14) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.8835.30; Wed, 18 Jun 2025 03:34:33 +0000 Received: from DS0PR11MB7529.namprd11.prod.outlook.com ([fe80::d244:15cd:1060:941a]) by DS0PR11MB7529.namprd11.prod.outlook.com ([fe80::d244:15cd:1060:941a%7]) with mapi id 15.20.8835.027; Wed, 18 Jun 2025 03:34:33 +0000 Message-ID: <6a9f95b4-3e56-4ed3-b2a2-c41a6f728eb2@intel.com> Date: Wed, 18 Jun 2025 11:40:38 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH rfcv3 15/21] intel_iommu: Bind/unbind guest page table to host To: Jason Gunthorpe CC: Nicolin Chen , "Duan, Zhenzhong" , Peter Xu , "qemu-devel@nongnu.org" , "alex.williamson@redhat.com" , "clg@redhat.com" , "eric.auger@redhat.com" , "mst@redhat.com" , "jasowang@redhat.com" , "ddutile@redhat.com" , "shameerali.kolothum.thodi@huawei.com" , "joao.m.martins@oracle.com" , "clement.mathieu--drif@eviden.com" , "Tian, Kevin" , "Peng, Chao P" , Yi Sun , Marcel Apfelbaum , Paolo Bonzini , Richard Henderson , Eduardo Habkost References: <0f8087f4-0c97-440d-84d2-f3f017f81041@intel.com> <29f5f434-1fe3-4b5e-91d1-f153e1e98602@intel.com> <20250617123707.GW1174925@nvidia.com> <20250617131113.GX1174925@nvidia.com> Content-Language: en-US From: Yi Liu In-Reply-To: <20250617131113.GX1174925@nvidia.com> Content-Type: text/plain; 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envelope-from=yi.l.liu@intel.com; helo=mgamail.intel.com X-Spam_score_int: -62 X-Spam_score: -6.3 X-Spam_bar: ------ X-Spam_report: (-6.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.89, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2025/6/17 21:11, Jason Gunthorpe wrote: > On Tue, Jun 17, 2025 at 09:03:32PM +0800, Yi Liu wrote: >>> I suggest fixing the Linux driver to refuse to run in sm_on mode if >>> the HW supports scalable mode and ecap_slts = false. That may not be >>> 100% spec compliant but it seems like a reasonable approach. >> >> running sm_on with only ecap_flts==true is what we want here. We want >> the guest use stage-1 page table hence it can be used by hw under the >> nested translation mode. While this page table is only available in sm_on >> mode. >> >> If we want to drop the legacy mode usage in virtualization environment, we >> might let linux iommu driver refuse running legacy mode while ecap_slts is >> false. I suppose HW is going to advertise both ecap_slts and ecap_flts. So >> this will just let guest get rid of using legacy mode. >> >> But this is not necessary so far. As the discussion going here, we intend >> to reuse the GPA HWPT allocated by VFIO container as well.[1] This is now >> aligned with Nic and Shameer. > > I think it is an issue, nobody really wants to accidently start > supporting and using shadow mode just because the VM is misconfigured. hmmm. intel iommu driver makes sm_on by default since v5.15. So if guest configs sm_off, that means it wants it. For the kernel <5.15, yes it will use legacy mode if it has not configured sm_on explicitly. So this seems not an issue. Actually, as I explained in the first hunk of [1], there is no issue with the legacy mode support. :) [1] https://lore.kernel.org/qemu-devel/20250521111452.3316354-1-zhenzhong.duan@intel.com/T/#m4c8fa70742001d4c22b3c297e240a2151d2c617f > What is desirable is to make this automatic and ensure we stay in the > nesting configuration only. yes, once QEMU supports nested translation based vIOMMU, it's better to use sm_on instead of legacy. I think for the kernels >= 5.15, this automation has already been achieved since sm is default on. >>> ARM is cleaner because it doesn't have these drivers issues. qemu can >>> reliably say not to use the S2 and all the existing guest kernels will >>> obey that. >> >> out of curious, does SMMU have legacy mode or a given version of SMMU >> only supports either legacy mode or newer mode? > > The SMMUv3 spec started out with definitions for S1 and S2 as well as > capability bits for them at day 0. So it never had this backward > compatible problem where we want to remove something that was > a mandatory part of the specification. got it. yes, it's all about backward compatible support. Regards, Yi Liu