From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 41XBHj6McPzDqF8 for ; Fri, 20 Jul 2018 23:17:05 +1000 (AEST) Date: Fri, 20 Jul 2018 16:16:57 +0300 From: "Michael S. Tsirkin" To: Anshuman Khandual Cc: virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, aik@ozlabs.ru, robh@kernel.org, joe@perches.com, elfring@users.sourceforge.net, david@gibson.dropbear.id.au, jasowang@redhat.com, benh@kernel.crashing.org, mpe@ellerman.id.au, hch@infradead.org, linuxram@us.ibm.com, haren@linux.vnet.ibm.com, paulus@samba.org, srikar@linux.vnet.ibm.com Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices Message-ID: <20180720161541-mutt-send-email-mst@kernel.org> References: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20180720035941.6844-1-khandual@linux.vnet.ibm.com> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Fri, Jul 20, 2018 at 09:29:37AM +0530, Anshuman Khandual wrote: > This patch series is the follow up on the discussions we had before about > the RFC titled [RFC,V2] virtio: Add platform specific DMA API translation > for virito devices (https://patchwork.kernel.org/patch/10417371/). There > were suggestions about doing away with two different paths of transactions > with the host/QEMU, first being the direct GPA and the other being the DMA > API based translations. > > First patch attempts to create a direct GPA mapping based DMA operations > structure called 'virtio_direct_dma_ops' with exact same implementation > of the direct GPA path which virtio core currently has but just wrapped in > a DMA API format. Virtio core must use 'virtio_direct_dma_ops' instead of > the arch default in absence of VIRTIO_F_IOMMU_PLATFORM flag to preserve the > existing semantics. The second patch does exactly that inside the function > virtio_finalize_features(). The third patch removes the default direct GPA > path from virtio core forcing it to use DMA API callbacks for all devices. > Now with that change, every device must have a DMA operations structure > associated with it. The fourth patch adds an additional hook which gives > the platform an opportunity to do yet another override if required. This > platform hook can be used on POWER Ultravisor based protected guests to > load up SWIOTLB DMA callbacks to do the required (as discussed previously > in the above mentioned thread how host is allowed to access only parts of > the guest GPA range) bounce buffering into the shared memory for all I/O > scatter gather buffers to be consumed on the host side. > > Please go through these patches and review whether this approach broadly > makes sense. I will appreciate suggestions, inputs, comments regarding > the patches or the approach in general. Thank you. I like how patches 1-3 look. Could you test performance with/without to see whether the extra indirection through use of DMA ops causes a measurable slow-down? > Anshuman Khandual (4): > virtio: Define virtio_direct_dma_ops structure > virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively > virtio: Force virtio core to use DMA API callbacks for all virtio devices > virtio: Add platform specific DMA API translation for virito devices > > arch/powerpc/include/asm/dma-mapping.h | 6 +++ > arch/powerpc/platforms/pseries/iommu.c | 6 +++ > drivers/virtio/virtio.c | 72 ++++++++++++++++++++++++++++++++++ > drivers/virtio/virtio_pci_common.h | 3 ++ > drivers/virtio/virtio_ring.c | 65 +----------------------------- > 5 files changed, 89 insertions(+), 63 deletions(-) > > -- > 2.9.3