On 07/20/2018 06:46 PM, Michael S. Tsirkin wrote:

On Fri, Jul 20, 2018 at 09:29:37AM +0530, Anshuman Khandual wrote:

quoted

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?

I ran this simple DD command 10 times where /dev/vda is a virtio block
device of 10GB size.

dd if=/dev/zero of=/dev/vda bs=8M count=1024 oflag=direct

With and without patches bandwidth which has a bit wide range does not
look that different from each other.

Without patches
===============

---------- 1 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.95557 s, 4.4 GB/s
---------- 2 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 2.05176 s, 4.2 GB/s
---------- 3 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.88314 s, 4.6 GB/s
---------- 4 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.84899 s, 4.6 GB/s
---------- 5 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 5.37184 s, 1.6 GB/s
---------- 6 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.9205 s, 4.5 GB/s
---------- 7 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 6.85166 s, 1.3 GB/s
---------- 8 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.74049 s, 4.9 GB/s
---------- 9 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 6.31699 s, 1.4 GB/s
---------- 10 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 2.47057 s, 3.5 GB/s


With patches
============

---------- 1 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 2.25993 s, 3.8 GB/s
---------- 2 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.82438 s, 4.7 GB/s
---------- 3 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.93856 s, 4.4 GB/s
---------- 4 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.83405 s, 4.7 GB/s
---------- 5 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 7.50199 s, 1.1 GB/s
---------- 6 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 2.28742 s, 3.8 GB/s
---------- 7 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 5.74958 s, 1.5 GB/s
---------- 8 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 1.99149 s, 4.3 GB/s
---------- 9 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 5.67647 s, 1.5 GB/s
---------- 10 ---------
1024+0 records in
1024+0 records out
8589934592 bytes (8.6 GB, 8.0 GiB) copied, 2.93957 s, 2.9 GB/s

Does this look okay ?