linux-kernel - Re: [PATCH v2 15/15] Drivers: hv: Add modules to expose /dev/mshv to VMMs running on Hyper-V

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Message-ID: <0909fb87-9370-4c59-b7ea-9ca673d456b3@linux.microsoft.com>
Date:   Fri, 25 Aug 2023 11:41:35 -0700
From:   Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To:     Boqun Feng <boqun.feng@...il.com>
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, wei.liu@...nel.org,
        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 v2 15/15] Drivers: hv: Add modules to expose /dev/mshv to
 VMMs running on Hyper-V

On 8/24/2023 11:31 AM, Boqun Feng wrote:
> On Thu, Aug 17, 2023 at 03:01:51PM -0700, Nuno Das Neves wrote:
> [...]
>> +static long
>> +mshv_dev_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>> +{
>> +	switch (ioctl) {
>> +	case MSHV_CHECK_EXTENSION:
>> +		return mshv_ioctl_check_extension((void __user *)arg);
>> +	case MSHV_CREATE_PARTITION:
>> +		return mshv.create_partition((void __user *)arg);
>> +	case MSHV_CREATE_VTL:
>> +		return mshv.create_vtl((void __user *)arg);
>> +	}
>> +
> 
> Shouldn't we also have a MSHV_GET_API_VERSION ioctl similar as KVM? So
> that in the future when we change these ioctl interfaces or semantics,
> we can bump up the API version to avoid breaking userspace?
> 

Note that we contribute and maintain support for this driver in
Cloud Hypervisor, so we control both sides of this API - if we break
userspace we can fix it ourselves.

For now the MSHV_CHECK_EXTENSION ioctl is sufficient - we can pass it
MSHV_CAP_CORE_API_STABLE, and it will return 0 to indicate that the API
is not yet stable.

A version check may be useful in the future, but is not needed right now.

Thanks
Nuno Das Neves