[PATCH v3 0/7] use vfio_dma_rw to read/write IOVAs from CPU side

Yan Zhao
2020-02-24T08:55:00Z

It is better for a device model to use IOVAs to read/write memory to
perform some sort of virtual DMA on behalf of the device.

patch 1 exports VFIO group to external user so that it can hold the group
reference until finishing using of it. It saves ~500 cycles that are spent
on VFIO group looking up, referencing and dereferencing. (this data is
measured with 1 VFIO user).

patch 2 introduces interface vfio_dma_rw().

patch 3 introduces interfaces vfio_pin_pages_from_group() and
vfio_unpin_pages_from_group() to get rid of VFIO group looking-up in
vfio_pin_pages() and vfio_unpin_pages().

patch 4-5 let kvmgt switch from calling kvm_read/write_guest() to calling
vfio_dma_rw to rw IOVAs.

patch 6 let kvmgt switch to use lighter version of vfio_pin/unpin_pages(),
i.e. vfio_pin/unpin_pages_from_group()

patch 7 enables kvmgt to read/write IOVAs of size larger than PAGE_SIZE.

Performance:

Comparison between vfio_dma_rw() and kvm_read/write_guest():

1. avergage CPU cycles of each interface measured with 1 running VM:
--------------------------------------------------
| rw | avg cycles of |
| size | (vfio_dma_rw - kvm_read/write_guest) |
|---------- ---------------------------------------|
| <= 1 page | +155 ~ +195 |
|--------------------------------------------------|
| 5 pages | -530 |
|--------------------------------------------------|
| 20 pages | -2005 ~ -2533 |
--------------------------------------------------

2. average scores

base: base code before applying code in this series. use
kvm_read/write_pages() to rw IOVAs

base + this series: use vfio_dma_rw() to read IOVAs and use
vfio_pin/unpin_pages_from_group(), and kvmgt is able to rw several pages
at a time.

Scores of benchmarks running in 1 VM each:
-----------------------------------------------------------------
| | glmark2 | lightsmark | openarena | heavens |
|-----------------------------------------------------------------|
| base | 1248 | 219.70 | 114.9 | 560 |
|-----------------------------------------------------------------|
|base + this series | 1248 | 225.8 | 115 | 559 |
-----------------------------------------------------------------

Sum of scores of two benchmark instances running in 2 VMs each:
-------------------------------------------------------
| | glmark2 | lightsmark | openarena |
|-------------------------------------------------------|
| base | 812 | 211.46 | 115.3 |
|-------------------------------------------------------|
|base + this series | 814 | 214.69 | 115.9 |
-------------------------------------------------------

Changelogs:
v2 --> v3:
- add vfio_group_get_external_user_from_dev() to improve performance (Alex)
- add vfio_pin/unpin_pages_from_group() to avoid repeated looking up of
VFIO group in vfio_pin/unpin_pages() (Alex)
- add a check for IOMMU_READ permission. (Alex)
- rename vfio_iommu_type1_rw_dma_nopin() to
vfio_iommu_type1_dma_rw_chunk(). (Alex)
- in kvmgt, change "write ? vfio_dma_rw(...,true) :
vfio_dma_rw(...,false)" to vfio_dma_rw(dev, gpa, buf, len, write)
(Alex and Paolo)
- in kvmgt, instead of read/write context pages 1:1, combining the
reads/writes of continuous IOVAs to take advantage of vfio_dma_rw() for
faster crossing page boundary accesses.

v1 --> v2:
- rename vfio_iova_rw to vfio_dma_rw, vfio iommu driver ops .iova_rw
to .dma_rw. (Alex).
- change iova and len from unsigned long to dma_addr_t and size_t,
respectively. (Alex)
- fix possible overflow in dma->vaddr + iova - dma->iova + offset (Alex)
- split DMAs from on page boundary to on max available size to eliminate
redundant searching of vfio_dma and switching mm. (Alex)
- add a check for IOMMU_WRITE permission.

Yan Zhao (7):
vfio: allow external user to get vfio group from device
vfio: introduce vfio_dma_rw to read/write a range of IOVAs
vfio: avoid inefficient lookup of VFIO group in vfio_pin/unpin_pages
drm/i915/gvt: hold reference of VFIO group during opening of vgpu
drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw
drm/i915/gvt: avoid unnecessary lookup in each vfio pin & unpin pages
drm/i915/gvt: rw more pages a time for shadow context

drivers/gpu/drm/i915/gvt/gvt.h | 1 +
drivers/gpu/drm/i915/gvt/kvmgt.c | 43 +++----
drivers/gpu/drm/i915/gvt/scheduler.c | 101 +++++++++++-----
drivers/vfio/vfio.c | 175 +++++++++++++++++++++++++++
drivers/vfio/vfio_iommu_type1.c | 77 ++++++++++++
include/linux/vfio.h | 13 ++
6 files changed, 358 insertions(+), 52 deletions(-)

--
2.17.1