Re: [RFC 0/4] Virtio uses DMA API for all devices

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Aug 03, 2018 at 10:41:41AM +0800, Jason Wang wrote:
> 
> 
> On 2018年08月03日 04:55, Michael S. Tsirkin wrote:
> > 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.
> > Jason did some work on profiling this. Unfortunately he reports
> > about 4% extra overhead from this switch on x86 with no vIOMMU.
> 
> The test is rather simple, just run pktgen (pktgen_sample01_simple.sh) in
> guest and measure PPS on tap on host.
> 
> Thanks

Could you supply host configuration involved please?

> > 
> > I expect he's writing up the data in more detail, but
> > just wanted to let you know this would be one more
> > thing to debug before we can just switch to DMA APIs.
> > 
> > 
> > > 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