Re: [PATCH v3 15/15] Drivers: hv: Add modules to expose /dev/mshv to VMMs running on Hyper-V

[Nuno Das Neves]

On 9/26/2023 1:03 AM, Greg KH wrote:
> On Tue, Sep 26, 2023 at 07:00:51AM +0000, Wei Liu wrote:
> > On Tue, Sep 26, 2023 at 08:31:03AM +0200, Greg KH wrote:
> > > On Tue, Sep 26, 2023 at 05:54:34AM +0000, Wei Liu wrote:
> > > > On Tue, Sep 26, 2023 at 06:52:46AM +0200, Greg KH wrote:
> > > > > On Mon, Sep 25, 2023 at 05:07:24PM -0700, Nuno Das Neves wrote:
> > > > > > On 9/23/2023 12:58 AM, Greg KH wrote:
> > > > > > > Also, drivers should never call pr_*() calls, always use the proper
> > > > > > > dev_*() calls instead.
> > > > > >
> > > > > > We only use struct device in one place in this driver, I think that is the
> > > > > > only place it makes sense to use dev_*() over pr_*() calls.
> > > > >
> > > > > Then the driver needs to be fixed to use struct device properly so that
> > > > > you do have access to it when you want to print messages.  That's a
> > > > > valid reason to pass around your device structure when needed.

What is the tangible benefit of using dev_*() over pr_*()? As I said,
our use of struct device is very limited compared to all the places we
may need to log errors.

pr_*() is used by many, many drivers; it seems to be the norm. We can
certainly add a pr_fmt to improve the logging.

> > > > Greg, ACRN and Nitro drivers do not pass around the device structure.
> > > > Instead, they rely on a global struct device. We can follow the same.
> > >
> > > A single global struct device is wrong, please don't do that.
> > >
> > > Don't copy bad design patterns from other drivers, be better :)

What makes it a bad pattern? It seems to be well-established, and is
also used by KVM which this driver is loosely modeled after:
https://elixir.bootlin.com/linux/v6.5.5/source/virt/kvm/kvm_main.c#L5128

> > If we're working with real devices like network cards or graphics cards
> > I would agree -- it is easy to imagine that we have several cards of the
> > same model in the system -- but in real world there won't be two
> > hypervisor instances running on the same hardware.
> >
> > We can stash the struct device inside some private data fields, but that
> > doesn't change the fact that we're still having one instance of the
> > structure. Is this what you want? Or do you have something else in mind?
>
> You have a real device, it's how userspace interacts with your
> subsystem.  Please use that, it is dynamically created and handled and
> is the correct representation here.

Are you referring to the struct device we get from calling
misc_register? If not, please be more specific.

How would you suggest we get a reference to that device via e.g. open()
or ioctl() without keeping a global reference to it?


Thanks,
Nuno