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

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 ( 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 (
in guest and measure PPS on tap on host.


> 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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