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

[Jason Wang]

On 2018å08æ03æ 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

> 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