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

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Message-ID: <05119cbc-155d-47c5-ab21-e6a08eba5dc4@linux.microsoft.com>
Date:   Tue, 26 Sep 2023 14:52:36 -0700
From:   Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To:     Greg KH <gregkh@...uxfoundation.org>, Wei Liu <wei.liu@...nel.org>
Cc:     linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-arch@...r.kernel.org, patches@...ts.linux.dev,
        mikelley@...rosoft.com, kys@...rosoft.com, haiyangz@...rosoft.com,
        decui@...rosoft.com, apais@...ux.microsoft.com,
        Tianyu.Lan@...rosoft.com, ssengar@...ux.microsoft.com,
        mukeshrathor@...rosoft.com, stanislav.kinsburskiy@...il.com,
        jinankjain@...ux.microsoft.com, vkuznets@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, will@...nel.org,
        catalin.marinas@....com
Subject: Re: [PATCH v3 15/15] Drivers: hv: Add modules to expose /dev/mshv to
 VMMs running on Hyper-V

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