[v2,2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups
Commit Message
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@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
---
v2: new patch.
virt/kvm/vfio.c | 25 +++++++++----------------
1 file changed, 9 insertions(+), 16 deletions(-)
Comments
On Fri, 14 Jul 2023 11:37:57 -0700
Dmitry Torokhov <dmitry.torokhov@gmail.com> wrote:
> 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@redhat.com>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> ---
>
> 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;
Nit, let's initialize ret = 0 when it's declared to avoid this. Series
looks good to me otherwise. Thanks,
Alex
> +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;
> }
>
@@ -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;
}