linux-kernel - Re: [RFC v1 7/8] mshv: implement in-kernel device framework

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Date:   Fri, 9 Jul 2021 16:32:48 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     Linux on Hyper-V List <linux-hyperv@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux Kernel List <linux-kernel@...r.kernel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Sunil Muthuswamy <sunilmut@...rosoft.com>,
        Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
        kumarpraveen@...ux.microsoft.com, pasha.tatashin@...een.com,
        Jonathan Corbet <corbet@....net>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lillian Grassin-Drake <ligrassi@...rosoft.com>,
        Muminul Islam <muislam@...rosoft.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [RFC v1 7/8] mshv: implement in-kernel device framework

On Fri, Jul 09, 2021 at 01:50:13PM +0000, Wei Liu wrote:
> On Fri, Jul 09, 2021 at 02:02:04PM +0100, Matthew Wilcox wrote:
> > On Fri, Jul 09, 2021 at 11:43:38AM +0000, Wei Liu wrote:
> > > +static long
> > > +mshv_partition_ioctl_create_device(struct mshv_partition *partition,
> > > +	void __user *user_args)
> > > +{
> > [...]
> > > +	mshv_partition_get(partition);
> > > +	r = anon_inode_getfd(ops->name, &mshv_device_fops, dev, O_RDWR | O_CLOEXEC);
> > > +	if (r < 0) {
> > > +		mshv_partition_put_no_destroy(partition);
> > > +		list_del(&dev->partition_node);
> > > +		ops->destroy(dev);
> > > +		goto out;
> > > +	}
> > > +
> > > +	cd->fd = r;
> > > +	r = 0;
> > 
> > Why return the fd in memory instead of returning the fd as the return
> > value from the ioctl?
> > 
> > > +	if (copy_to_user(user_args, &tmp, sizeof(tmp))) {
> > > +		r = -EFAULT;
> > > +		goto out;
> > > +	}
> > 
> > ... this could then disappear.
> 
> Thanks for your comment, Matthew.
> 
> This is intentionally because I didn't want to deviate from KVM's API.
> The fewer differences the better.

Then don't define your own structure.  Use theirs.