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Mon, 28 Mar 2022 10:17:25 -0700 (PDT) Received: from redhat.com ([38.15.36.239]) by smtp.gmail.com with ESMTPSA id c6-20020a056e020bc600b002c6731e7cb8sm7563658ilu.31.2022.03.28.10.17.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 28 Mar 2022 10:17:25 -0700 (PDT) Date: Mon, 28 Mar 2022 11:17:23 -0600 From: Alex Williamson To: Jason Gunthorpe Subject: Re: [PATCH RFC 08/12] iommufd: IOCTLs for the io_pagetable Message-ID: <20220328111723.24fa5118.alex.williamson@redhat.com> In-Reply-To: <20220324134622.GB1184709@nvidia.com> References: <0-v1-e79cd8d168e8+6-iommufd_jgg@nvidia.com> <8-v1-e79cd8d168e8+6-iommufd_jgg@nvidia.com> <20220323131038.3b5cb95b.alex.williamson@redhat.com> <20220323193439.GS11336@nvidia.com> <20220323140446.097fd8cc.alex.williamson@redhat.com> <20220323203418.GT11336@nvidia.com> <20220323225438.GA1228113@nvidia.com> <20220324134622.GB1184709@nvidia.com> Organization: Red Hat MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=alex.williamson@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Cc: Jean-Philippe Brucker , "Tian, Kevin" , "kvm@vger.kernel.org" , "Michael S. Tsirkin" , Jason Wang , Cornelia Huck , Niklas Schnelle , Chaitanya Kulkarni , Daniel Jordan , "iommu@lists.linux-foundation.org" , "Martins, Joao" , David Gibson X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Thu, 24 Mar 2022 10:46:22 -0300 Jason Gunthorpe wrote: > On Thu, Mar 24, 2022 at 07:25:03AM +0000, Tian, Kevin wrote: > > > Based on that here is a quick tweak of the force-snoop part (not compiled). > > I liked your previous idea better, that IOMMU_CAP_CACHE_COHERENCY > started out OK but got weird. So lets fix it back to the way it was. > > How about this: > > https://github.com/jgunthorpe/linux/commits/intel_no_snoop > > b11c19a4b34c2a iommu: Move the Intel no-snoop control off of IOMMU_CACHE > 5263947f9d5f36 vfio: Require that device support DMA cache coherence I have some issues with the argument here: This will block device/platform/iommu combinations that do not support cache coherent DMA - but these never worked anyhow as VFIO did not expose any interface to perform the required cache maintenance operations. VFIO never intended to provide such operations, it only tried to make the coherence of the device visible to userspace such that it can perform operations via other means, for example via KVM. The "never worked" statement here seems false. Commit b11c19a4b34c2a also appears to be a behavioral change. AIUI vfio_domain.enforce_cache_coherency would only be set on Intel VT-d where snoop-control is supported, this translates to KVM emulating coherency instructions everywhere except VT-d w/ snoop-control. My understanding of AMD-Vi is that no-snoop TLPs are always coherent, so this would trigger unnecessary wbinvd emulation on those platforms. I don't know if other archs need similar, but it seems we're changing polarity wrt no-snoop TLPs from "everyone is coherent except this case on Intel" to "everyone is non-coherent except this opposite case on Intel". Thanks, Alex > eab4b381c64a30 iommu: Restore IOMMU_CAP_CACHE_COHERENCY to its original meaning > 2752e12bed48f6 iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma_coherent() > > If you like it could you take it from here? > > Thanks, > Jason > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu