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

Jason Wang
Fri, 03 Aug 2018 02:41:41 -0000

On 2018?08?03? 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