[PATCH v2 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups

From: Dmitry Torokhov
Date: Fri Jul 14 2023 - 14:38:12 EST


Stop taking kv->lock mutex in kvm_vfio_update_coherency() and instead
call it with this mutex held: the callers of the function usually
already have it taken (and released) before calling
kvm_vfio_update_coherency(). This avoid bouncing the lock up and down.

The exception is kvm_vfio_release() where we do not take the lock, but
it is being executed when the very last reference to kvm_device is being
dropped, so there are no concerns about concurrency.

Suggested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
---

v2: new patch.

virt/kvm/vfio.c | 25 +++++++++----------------
1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index cd46d7ef98d6..9868e7ccb5fb 100644
--- a/virt/kvm/vfio.c
+++ b/virt/kvm/vfio.c
@@ -122,8 +122,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
bool noncoherent = false;
struct kvm_vfio_group *kvg;

- mutex_lock(&kv->lock);
-
list_for_each_entry(kvg, &kv->group_list, node) {
if (!kvm_vfio_file_enforced_coherent(kvg->file)) {
noncoherent = true;
@@ -139,8 +137,6 @@ static void kvm_vfio_update_coherency(struct kvm_device *dev)
else
kvm_arch_unregister_noncoherent_dma(dev->kvm);
}
-
- mutex_unlock(&kv->lock);
}

static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
@@ -157,7 +153,7 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
/* Ensure the FD is a vfio group FD.*/
if (!kvm_vfio_file_is_group(filp)) {
ret = -EINVAL;
- goto err_fput;
+ goto out_fput;
}

mutex_lock(&kv->lock);
@@ -165,30 +161,27 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
list_for_each_entry(kvg, &kv->group_list, node) {
if (kvg->file == filp) {
ret = -EEXIST;
- goto err_unlock;
+ goto out_unlock;
}
}

kvg = kzalloc(sizeof(*kvg), GFP_KERNEL_ACCOUNT);
if (!kvg) {
ret = -ENOMEM;
- goto err_unlock;
+ goto out_unlock;
}

- kvg->file = filp;
+ kvg->file = get_file(filp);
list_add_tail(&kvg->node, &kv->group_list);

kvm_arch_start_assignment(dev->kvm);
kvm_vfio_file_set_kvm(kvg->file, dev->kvm);
-
- mutex_unlock(&kv->lock);
-
kvm_vfio_update_coherency(dev);

- return 0;
-err_unlock:
+ ret = 0;
+out_unlock:
mutex_unlock(&kv->lock);
-err_fput:
+out_fput:
fput(filp);
return ret;
}
@@ -224,12 +217,12 @@ static int kvm_vfio_group_del(struct kvm_device *dev, unsigned int fd)
break;
}

+ kvm_vfio_update_coherency(dev);
+
mutex_unlock(&kv->lock);

fdput(f);

- kvm_vfio_update_coherency(dev);
-
return ret;
}

--
2.41.0.255.g8b1d071c50-goog