From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christoph Hellwig Subject: Re: [PATCH 0/3] Fix virtio-blk issue with SWIOTLB Date: Mon, 14 Jan 2019 21:19:35 +0100 Message-ID: <20190114201935.GA10781@lst.de> References: <20190110134433.15672-1-joro@8bytes.org> <5ae1341e-62ec-0478-552b-259eabf9fb17@redhat.com> <20190111091502.GC5825@8bytes.org> <38bcbd46-674c-348a-cbd6-66bd431e986a@redhat.com> <20190114095002.GA29874@lst.de> <20190114131114-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20190114131114-mutt-send-email-mst@kernel.org> Sender: linux-kernel-owner@vger.kernel.org To: "Michael S. Tsirkin" Cc: Jason Wang , Christoph Hellwig , Joerg Roedel , Konrad Rzeszutek Wilk , Jens Axboe , virtualization@lists.linux-foundation.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, jfehlig@suse.com, jon.grimm@amd.com, brijesh.singh@amd.com List-Id: iommu@lists.linux-foundation.org On Mon, Jan 14, 2019 at 01:20:45PM -0500, Michael S. Tsirkin wrote: > I don't think so - the issue is really that DMA API does not yet handle > the SEV case 100% correctly. I suspect passthrough devices would have > the same issue. The DMA API handles the SEV case perfectly. Its just that virtio_blk supports huge segments that virtio does not generally support, but that is not related to SEV in any way. > In fact whoever sets IOMMU_PLATFORM is completely unaffected by > Christoph's pet peeve. No, the above happens only when we set IOMMU_PLATFORM. > Christoph is saying that !IOMMU_PLATFORM devices should hide the > compatibility code in a special per-device DMA API implementation. > Which would be fine especially if we can manage not to introduce a bunch > of indirect calls all over the place and hurt performance. It's just > that the benefit is unlikely to be big (e.g. we can't also get rid of > the virtio specific memory barriers) so no one was motivated enough to > work on it. No. The problem is that we still haven't fully specified what IOMMU_PLATFORM and !IOMMU_PLATFORM actually mean. Your "ACCESS_PLATFORM/ORDER_PLATFORM" commit in the virtio-spec repo improves it a little bit, but it is still far from enough. As a start VIRTIO_F_ORDER_PLATFORM and VIRTIO_F_ACCESS_PLATFORM absolutely MUST be set for hardware implementations. Otherwise said hardware has no chance of working on anything but the most x86-like systems. Second software implementations SHOULD set VIRTIO_F_ACCESS_PLATFORM, because otherwise we can't add proper handling for things like SEV or the IBM "secure hypervisor" thing. Last but not least a lot of wording outside the area describing these flags really needs some major updates in terms of DMA access.