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

[Anshuman Khandual]

On 07/20/2018 06:46 PM, 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.
> 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 ?