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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 2 Aug 2018 23:55:39 +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.

Jason did some work on profiling this. Unfortunately he reports
about 4% extra overhead from this switch on x86 with no vIOMMU.

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