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Message-ID: <20230714224538.404793-2-dmitry.torokhov@gmail.com>
Date: Fri, 14 Jul 2023 15:45:33 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Alex Williamson <alex.williamson@...hat.com>,
Greg KH <gregkh@...uxfoundation.org>,
Sean Christopherson <seanjc@...gle.com>,
Roxana Bradescu <roxabee@...gle.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/2] kvm/vfio: avoid bouncing the mutex when adding and deleting groups
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@...hat.com>
Reviewed-by: Alex Williamson <alex.williamson@...hat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
---
v3: initialize "ret" with 0 (per Alex), added Alex's reviewed-by
v2: new patch.
virt/kvm/vfio.c | 26 +++++++++-----------------
1 file changed, 9 insertions(+), 17 deletions(-)
diff --git a/virt/kvm/vfio.c b/virt/kvm/vfio.c
index cd46d7ef98d6..dbf2b855cf78 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)
@@ -148,7 +144,7 @@ static int kvm_vfio_group_add(struct kvm_device *dev, unsigned int fd)
struct kvm_vfio *kv = dev->private;
struct kvm_vfio_group *kvg;
struct file *filp;
- int ret;
+ int ret = 0;
filp = fget(fd);
if (!filp)
@@ -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 +161,26 @@ 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:
+out_unlock:
mutex_unlock(&kv->lock);
-err_fput:
+out_fput:
fput(filp);
return ret;
}
@@ -224,12 +216,12 @@ 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
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