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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 20 Jul 2018 16:16:57 +0300

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.

I like how patches 1-3 look. Could you test performance
with/without to see whether the extra indirection through
use of DMA ops causes a measurable slow-down?

> 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