linux-kernel - Re: [PATCH] vfio/mdev: Check globally for duplicate devices

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Date:   Wed, 16 May 2018 16:38:17 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kwankhede@...dia.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vfio/mdev: Check globally for duplicate devices

On Wed, 16 May 2018 08:23:20 -0600
Alex Williamson <alex.williamson@...hat.com> wrote:

> On Wed, 16 May 2018 11:05:06 +0200
> Cornelia Huck <cohuck@...hat.com> wrote:
> 
> > On Tue, 15 May 2018 14:17:04 -0600
> > Alex Williamson <alex.williamson@...hat.com> wrote:
> >   
> > > When we create an mdev device, we check for duplicates against the
> > > parent device and return -EEXIST if found, but the mdev device
> > > namespace is global since we'll link all devices from the bus.  We do
> > > catch this later in sysfs_do_create_link_sd() to return -EEXIST, but
> > > with it comes a kernel warning and stack trace for trying to create
> > > duplicate sysfs links, which makes it an undesirable response.
> > > 
> > > Therefore we should really be looking for duplicates across all mdev
> > > parent devices, or as implemented here, against our mdev device list.
> > > 
> > > Notably, mdev_parent.lock really only seems to be serializing device
> > > creation and removal per parent.  I'm not sure if this is necessary,
> > > mdev vendor drivers could easily provide this serialization if it
> > > is required, but a side-effect of holding the mdev_list_lock to
> > > protect the namespace is actually greater serialization across the
> > > create and remove paths, so mdev_parent.lock is removed.  If we can
> > > show that vendor drivers handle the create/remove paths themselves,
> > > perhaps we can refine the locking granularity.    
> > 
> > I'm not sure whether more locking granularity on the create/remove
> > paths is really worth the effort.  
> 
> Perhaps not, but I thought I should at least mention it as a
> consideration.
> 
> > > Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> > > ---
> > >  drivers/vfio/mdev/mdev_core.c    |   79 ++++++++++----------------------------
> > >  drivers/vfio/mdev/mdev_private.h |    1 
> > >  2 files changed, 20 insertions(+), 60 deletions(-)    
> > 
> > In general, I think this patch makes sense; some nits below.
> >   
> > > 
> > > diff --git a/drivers/vfio/mdev/mdev_core.c b/drivers/vfio/mdev/mdev_core.c
> > > index 126991046eb7..3d8898a2baaf 100644
> > > --- a/drivers/vfio/mdev/mdev_core.c
> > > +++ b/drivers/vfio/mdev/mdev_core.c    
> >   
> > > @@ -376,12 +346,13 @@ int mdev_device_remove(struct device *dev, bool force_remove)
> > >  	struct mdev_device *mdev, *tmp;
> > >  	struct mdev_parent *parent;
> > >  	struct mdev_type *type;
> > > -	int ret;
> > > +	int ret = 0;    
> > 
> > I don't think you need to init this, as ret should either be set to
> > -ENODEV or the return code of mdev_device_remove_ops(), shouldn't it?  
> 
> Yep, I think this is a leftover from before I decided I should goto a
> common out, it's unnecessary now.  Removed.

OK, with that change

Reviewed-by: Cornelia Huck <cohuck@...hat.com>