Re: [PATCH v2 2/2] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_dma_rw

[Alex Williamson <alex.williamson@redhat.com>]

On Tue, 14 Jan 2020 22:54:55 -0500
Yan Zhao <yan.y.zhao@intel.com> 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?
 
> 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@intel.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  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);

Thanks,
Alex

>  }