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

    >
    > 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

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