Re: [PATCH v4 0/7] use vfio_dma_rw to read/write IOVAs from CPU side

Yan Zhao
2020-03-16T00:06:09Z

On Sat, Mar 14, 2020 at 06:29:58AM +0800, Alex Williamson wrote:
> [Cc +NVIDIA]
>
> On Thu, 12 Mar 2020 23:05:48 -0400
> Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> > 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_group_pin_pages() and
> > vfio_group_unpin_pages() 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_group_pin/unpin_pages()
> >
> > patch 7 enables kvmgt to read/write IOVAs of size larger than PAGE_SIZE.
>
> This looks pretty good to me, hopefully Kirti and Neo can find some
> advantage with this series as well. Given that we're also trying to
> get the migration interface and dirty page tracking integrated for
> v5.7, would it make sense to merge the first 3 patches via my next
> branch? This is probably the easiest way to update the dirty tracking.
> Thanks,
>
sure. glad to hear that :)

Thanks
Yan

>
> > 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_group_pin/unpin_pages(), 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:
> > v3 --> v4:
> > - rebased to 5.6.0-rc4+
> > - adjust wrap position for vfio_group_get_external_user_from_dev() in
> > header file.(Alex)
> > - updated function description of vfio_group_get_external_user_from_dev()
> > (Alex)
> > - fixed Error path group reference leaking in
> > vfio_group_get_external_user_from_dev() (Alex)
> > - reurn 0 for success or errno in vfio_dma_rw_chunk(). (Alex)
> > - renamed iova to user_iova in interface vfio_dam_rw().
> > - renamed vfio_pin_pages_from_group() and vfio_unpin_pages_from_group() to
> > vfio_group_pin_pages() and vfio_group_unpin_pages()
> > - renamed user_pfn to user_iova_pfn in vfio_group_pin_pages() and
> > vfio_group_unpin_pages()
> >
> > 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 operations on 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: switch to user vfio_group_pin/upin_pages
> > drm/i915/gvt: rw more pages a time for shadow context
> >
> > drivers/gpu/drm/i915/gvt/kvmgt.c | 46 ++++---
> > drivers/gpu/drm/i915/gvt/scheduler.c | 97 ++++++++++-----
> > drivers/vfio/vfio.c | 180 +++++++++++++++++++++++++++
> > drivers/vfio/vfio_iommu_type1.c | 76 +++++++++++
> > include/linux/vfio.h | 13 ++
> > 5 files changed, 360 insertions(+), 52 deletions(-)
> >
>