On Thu, Jan 16, 2020 at 04:06:51AM +0800, Alex Williamson wrote:
> On Tue, 14 Jan 2020 22:54:55 -0500
> Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
>
> > As a device model, it is better to read/write guest memory using vfio
> > interface, so that vfio is able to maintain dirty info of device IOVAs.
> >
> > Compared to kvm interfaces kvm_read/write_guest(), vfio_dma_rw() has ~600
> > cycles more overhead on average.
> >
> > -------------------------------------
> > | interface | avg cpu cycles |
> > |-----------------------------------|
> > | kvm_write_guest | 1554 |
> > | ----------------------------------|
> > | kvm_read_guest | 707 |
> > |-----------------------------------|
> > | vfio_dma_rw(w) | 2274 |
> > |-----------------------------------|
> > | vfio_dma_rw(r) | 1378 |
> > -------------------------------------
>
> In v1 you had:
>
> -------------------------------------
> | interface | avg cpu cycles |
> |-----------------------------------|
> | kvm_write_guest | 1546 |
> | ----------------------------------|
> | kvm_read_guest | 686 |
> |-----------------------------------|
> | vfio_iova_rw(w) | 2233 |
> |-----------------------------------|
> | vfio_iova_rw(r) | 1262 |
> -------------------------------------
>
> So the kvm numbers remained within +0.5-3% while the vfio numbers are
> now +1.8-9.2%. I would have expected the algorithm change to at least
> not be worse for small accesses and be better for accesses crossing
> page boundaries. Do you know what happened?
>
I only tested the 4 interfaces in GVT's environment, where most of the
guest memory accesses are less than one page.
And the different fluctuations should be caused by the locks.
vfio_dma_rw contends locks with other vfio accesses which are assumed to
be abundant in the case of GVT.
> > Comparison of benchmarks scores are as blow:
> > ------------------------------------------------------
> > | avg score | kvm_read/write_guest | vfio_dma_rw |
> > |----------------------------------------------------|
> > | Glmark2 | 1284 | 1296 |
> > |----------------------------------------------------|
> > | Lightsmark | 61.24 | 61.27 |
> > |----------------------------------------------------|
> > | OpenArena | 140.9 | 137.4 |
> > |----------------------------------------------------|
> > | Heaven | 671 | 670 |
> > ------------------------------------------------------
> > No obvious performance downgrade found.
> >
> > Cc: Kevin Tian <kevin.tian@xxxxxxxxx>
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/i915/gvt/kvmgt.c | 26 +++++++-------------------
> > 1 file changed, 7 insertions(+), 19 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > index bd79a9718cc7..17edc9a7ff05 100644
> > --- a/drivers/gpu/drm/i915/gvt/kvmgt.c
> > +++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
> > @@ -1966,31 +1966,19 @@ static int kvmgt_rw_gpa(unsigned long handle, unsigned long gpa,
> > void *buf, unsigned long len, bool write)
> > {
> > struct kvmgt_guest_info *info;
> > - struct kvm *kvm;
> > - int idx, ret;
> > - bool kthread = current->mm == NULL;
> > + int ret;
> > + struct intel_vgpu *vgpu;
> > + struct device *dev;
> >
> > if (!handle_valid(handle))
> > return -ESRCH;
> >
> > info = (struct kvmgt_guest_info *)handle;
> > - kvm = info->kvm;
> > -
> > - if (kthread) {
> > - if (!mmget_not_zero(kvm->mm))
> > - return -EFAULT;
> > - use_mm(kvm->mm);
> > - }
> > -
> > - idx = srcu_read_lock(&kvm->srcu);
> > - ret = write ? kvm_write_guest(kvm, gpa, buf, len) :
> > - kvm_read_guest(kvm, gpa, buf, len);
> > - srcu_read_unlock(&kvm->srcu, idx);
> > + vgpu = info->vgpu;
> > + dev = mdev_dev(vgpu->vdev.mdev);
> >
> > - if (kthread) {
> > - unuse_mm(kvm->mm);
> > - mmput(kvm->mm);
> > - }
> > + ret = write ? vfio_dma_rw(dev, gpa, buf, len, true) :
> > + vfio_dma_rw(dev, gpa, buf, len, false);
>
> As Paolo suggested previously, this can be simplified:
>
> ret = vfio_dma_rw(dev, gpa, buf, len, write);
>
> >
> > return ret;
>
> Or even more simple, remove the ret variable:
>
> return vfio_dma_rw(dev, gpa, buf, len, write);
>
oh, it seems that I missed Paolo's mail. will change it. thank you!
Thanks
Yan
>
> > }
>
