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Subject[PATCH v3 02/21] drm/i915/gvt: subsitute kvm_read/write_guest with vfio_iova_rw
Date
From: Yan Zhao <yan.y.zhao@intel.com>

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 CPU side interfaces kvm_read/write_guest(), vfio_iova_rw()
has ~600 cycles more overhead on average.
-------------------------------------
| interface | avg cpu cycles |
|-----------------------------------|
| kvm_write_guest | 1546 |
| ----------------------------------|
| kvm_read_guest | 686 |
|-----------------------------------|
| vfio_iova_rw(w) | 2233 |
|-----------------------------------|
| vfio_iova_rw(r) | 1262 |
-------------------------------------

Comparison of benchmarks scores are as blow:
---------------------------------------------------------
| avg score | kvm_read/write_guest | vfio_iova_rw |
---------------------------------------------------------
| Glmark2 | 1132 | 1138.2 |
---------------------------------------------------------
| Lightsmark | 61.558 | 61.538 |
|--------------------------------------------------------
| OpenArena | 142.77 | 136.6 |
---------------------------------------------------------
| Heaven | 698 | 686.8 |
--------------------------------------------------------
No obvious performance downgrade found.

Cc: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
[peterx: pass in "write" to vfio_iova_rw(), suggested by Paolo]
Signed-off-by: Peter Xu <peterx@redhat.com>
---
drivers/gpu/drm/i915/gvt/kvmgt.c | 25 ++++++-------------------
1 file changed, 6 insertions(+), 19 deletions(-)

diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
index 3259a1fa69e1..5fb82f285b98 100644
--- a/drivers/gpu/drm/i915/gvt/kvmgt.c
+++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
@@ -1968,31 +1968,18 @@ 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 = vfio_iova_rw(dev, gpa, buf, len, write);

return ret;
}
--
2.24.1
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