Messages in this thread | | | Date | Wed, 18 May 2022 11:33:37 -0400 | Subject | Re: [PATCH 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM | From | Matthew Rosato <> |
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On 5/18/22 11:12 AM, Jason Gunthorpe wrote: > On Wed, May 18, 2022 at 10:37:48AM -0400, Matthew Rosato wrote: >> On 5/17/22 2:08 PM, Matthew Rosato wrote: >>> Rather than relying on a notifier for associating the KVM with >>> the group, let's assume that the association has already been >>> made prior to device_open. The first time a device is opened >>> associate the group KVM with the device. >>> >>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com> >>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com> >> >> ... >> >>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c >>> index cfcff7764403..c5d421eda275 100644 >>> +++ b/drivers/vfio/vfio.c >>> @@ -10,6 +10,7 @@ >>> * Author: Tom Lyon, pugs@cisco.com >>> */ >>> +#include "linux/kvm_host.h" >>> #include <linux/cdev.h> >>> #include <linux/compat.h> >>> #include <linux/device.h> >>> @@ -1083,6 +1084,13 @@ static struct file *vfio_device_open(struct vfio_device *device) >>> mutex_lock(&device->dev_set->lock); >>> device->open_count++; >>> + down_write(&device->group->group_rwsem); >>> + if (device->open_count == 1 && device->group->kvm) { >>> + device->kvm = device->group->kvm; >>> + kvm_get_kvm(device->kvm); >> >> Did some more compile testing, since vfio has no hard kvm dependency, >> kvm_get_kvm and kvm_put_kvm are an issue if KVM is a module while vfio is >> built-in... > > Ugh, my other plan was to have the driver itself capture the kvm, ie > we lock the group_rwsem to keep the group->kvm valid and then pass the > kvm to open_device in some way, then the driver can kvm_get_kvm() it >
Hrm... If we did that we would have to re-evaluate some other usage of the rwsem e.g. if driver open_device calls vfio_register_iommu_notifier it will try to get the rwsem but it's already locked.
> Alternatively, I don't know why kvm_get_kvm() is an exported symbol > when it is just calling refcount_inc() - inlining it would be an > improvement I think. >
I think that would work for kvm_get_kvm, but kvm_put_kvm (which we also need) calls kvm_destroy_kvm after the refcount_dec and that can't be inlined
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