linux-kernel - Re: [PATCH 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET

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Message-ID: <34c5fd53-c75c-cb96-c627-9d30b8c45c37@linux.ibm.com>
Date:   Wed, 18 May 2022 11:33:37 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     alex.williamson@...hat.com, cohuck@...hat.com,
        borntraeger@...ux.ibm.com, jjherne@...ux.ibm.com,
        akrowiak@...ux.ibm.com, pasic@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com, hch@...radead.org,
        intel-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM

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@...dia.com>
>>> Signed-off-by: Matthew Rosato <mjrosato@...ux.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@...co.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