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Thu, 3 Jun 2021 13:09:28 +0000 Received: from BL0PR12MB5506.namprd12.prod.outlook.com ([fe80::3d51:a3b9:8611:684e]) by BL0PR12MB5506.namprd12.prod.outlook.com ([fe80::3d51:a3b9:8611:684e%6]) with mapi id 15.20.4195.024; Thu, 3 Jun 2021 13:09:28 +0000 Date: Thu, 3 Jun 2021 10:09:27 -0300 From: Jason Gunthorpe To: Jason Wang Cc: Alex Williamson , "Tian, Kevin" , Jean-Philippe Brucker , "Jiang, Dave" , "Raj, Ashok" , "kvm@vger.kernel.org" , Jonathan Corbet , Robin Murphy , LKML , "iommu@lists.linux-foundation.org" , David Gibson , Kirti Wankhede , David Woodhouse Subject: Re: [RFC] /dev/ioasid uAPI proposal Message-ID: <20210603130927.GZ1002214@nvidia.com> References: <20210602160140.GV1002214@nvidia.com> <20210602111117.026d4a26.alex.williamson@redhat.com> <20210602173510.GE1002214@nvidia.com> <20210602120111.5e5bcf93.alex.williamson@redhat.com> <20210602180925.GH1002214@nvidia.com> <20210602130053.615db578.alex.williamson@redhat.com> <20210602195404.GI1002214@nvidia.com> <20210602143734.72fb4fa4.alex.williamson@redhat.com> <6a9426d7-ed55-e006-9c4c-6b7c78142e39@redhat.com> Content-Type: text/plain; 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So for simplicity, we rules out the IOMMU that can't enforce coherency > in vhost-vDPA if the parent purely depends on the platform IOMMU: VDPA HW cannot issue no-snoop TLPs in the first place. virtio does not define a protocol to discover such a functionality, nor do any virtio drivers implement the required platform specific cache flushing to make no-snoop TLPs work. It is fundamentally part of the virtio HW PCI API that a device vendor cannot alter. Basically since we already know that the virtio kernel drivers do not call the cache flush instruction we don't need the weird KVM logic to turn it on at all. Enforcing no-snoop at the IOMMU here is redundant/confusing. 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charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Thu, Jun 03, 2021 at 10:52:51AM +0800, Jason Wang wrote: > Basically, we don't want to bother with pseudo KVM device like what VFIO > did. So for simplicity, we rules out the IOMMU that can't enforce coherency > in vhost-vDPA if the parent purely depends on the platform IOMMU: VDPA HW cannot issue no-snoop TLPs in the first place. virtio does not define a protocol to discover such a functionality, nor do any virtio drivers implement the required platform specific cache flushing to make no-snoop TLPs work. It is fundamentally part of the virtio HW PCI API that a device vendor cannot alter. Basically since we already know that the virtio kernel drivers do not call the cache flush instruction we don't need the weird KVM logic to turn it on at all. Enforcing no-snoop at the IOMMU here is redundant/confusing. Jason _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu