Message-ID: <6a26cbf8-7877-4f39-0ed3-7bbc306f9fe5@linux.microsoft.com>
Date: Fri, 5 Sep 2025 14:41:52 -0700
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>,
 Michael Kelley <mhklinux@...look.com>,
 "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
 "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
 "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
 "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
 "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
 "virtualization@...ts.linux.dev" <virtualization@...ts.linux.dev>
Cc: "maarten.lankhorst@...ux.intel.com" <maarten.lankhorst@...ux.intel.com>,
 "mripard@...nel.org" <mripard@...nel.org>,
 "tzimmermann@...e.de" <tzimmermann@...e.de>,
 "airlied@...il.com" <airlied@...il.com>, "simona@...ll.ch"
 <simona@...ll.ch>, "jikos@...nel.org" <jikos@...nel.org>,
 "bentiss@...nel.org" <bentiss@...nel.org>,
 "kys@...rosoft.com" <kys@...rosoft.com>,
 "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
 "wei.liu@...nel.org" <wei.liu@...nel.org>,
 "decui@...rosoft.com" <decui@...rosoft.com>,
 "dmitry.torokhov@...il.com" <dmitry.torokhov@...il.com>,
 "andrew+netdev@...n.ch" <andrew+netdev@...n.ch>,
 "davem@...emloft.net" <davem@...emloft.net>,
 "edumazet@...gle.com" <edumazet@...gle.com>,
 "kuba@...nel.org" <kuba@...nel.org>, "pabeni@...hat.com"
 <pabeni@...hat.com>, "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
 "James.Bottomley@...senPartnership.com"
 <James.Bottomley@...senPartnership.com>,
 "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
 "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
 "deller@....de" <deller@....de>, "arnd@...db.de" <arnd@...db.de>,
 "sgarzare@...hat.com" <sgarzare@...hat.com>,
 "horms@...nel.org" <horms@...nel.org>
Subject: Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly

On 9/5/25 13:08, Nuno Das Neves wrote:
> On 9/4/2025 11:18 AM, Mukesh R wrote:
>> On 9/4/25 09:26, Michael Kelley wrote:
>>> From: Mukesh R <mrathor@...ux.microsoft.com> Sent: Wednesday, September 3, 2025 7:17 PM
>>>>
>>>> On 9/2/25 07:42, Michael Kelley wrote:
>>>>> From: Mukesh Rathor <mrathor@...ux.microsoft.com> Sent: Wednesday, August 27, 2025 6:00 PM
>>>>>>
>>>>>> At present, drivers/Makefile will subst =m to =y for CONFIG_HYPERV for hv
>>>>>> subdir. Also, drivers/hv/Makefile replaces =m to =y to build in
>>>>>> hv_common.c that is needed for the drivers. Moreover, vmbus driver is
>>>>>> built if CONFIG_HYPER is set, either loadable or builtin.
>>>>>>
>>>>>> This is not a good approach. CONFIG_HYPERV is really an umbrella config that
>>>>>> encompasses builtin code and various other things and not a dedicated config
>>>>>> option for VMBUS. Vmbus should really have a config option just like
>>>>>> CONFIG_HYPERV_BALLOON etc. This small series introduces CONFIG_HYPERV_VMBUS
>>>>>> to build VMBUS driver and make that distinction explicit. With that
>>>>>> CONFIG_HYPERV could be changed to bool.
>>>>>
>>>>> Separating the core hypervisor support (CONFIG_HYPERV) from the VMBus
>>>>> support (CONFIG_HYPERV_VMBUS) makes sense to me. Overall the code
>>>>> is already mostly in separate source files code, though there's some
>>>>> entanglement in the handling of VMBus interrupts, which could be
>>>>> improved later.
>>>>>
>>>>> However, I have a compatibility concern. Consider this scenario:
>>>>>
>>>>> 1) Assume running in a Hyper-V VM with a current Linux kernel version
>>>>>     built with CONFIG_HYPERV=m.
>>>>> 2) Grab a new version of kernel source code that contains this patch set.
>>>>> 3) Run 'make olddefconfig' to create the .config file for the new kernel.
>>>>> 4) Build the new kernel. This succeeds.
>>>>> 5) Install and run the new kernel in the Hyper-V VM. This fails.
>>>>>
>>>>> The failure occurs because CONFIG_HYPERV=m is no longer legal,
>>>>> so the .config file created in Step 3 has CONFIG_HYPERV=n. The
>>>>> newly built kernel has no Hyper-V support and won't run in a
>>>>> Hyper-V VM.
> 
> It surprises me a little that =m doesn't get 'fixed up' to =y in this case.
> I guess any invalid value turns to =n, which makes sense most of the time.
> 
>>>>>
>>>>> As a second issue, if in Step 1 the current kernel was built with
>>>>> CONFIG_HYPERV=y, then the .config file for the new kernel will have
>>>>> CONFIG_HYPERV=y, which is better. But CONFIG_HYPERV_VMBUS
>>>>> defaults to 'n', so the new kernel doesn't have any VMBus drivers
>>>>> and won't run in a typical Hyper-V VM.
>>>>>
>>>>> The second issue could be fixed by assigning CONFIG_HYPERV_VMBUS
>>>>> a default value, such as whatever CONFIG_HYPERV is set to. But
>>>>> I'm not sure how to fix the first issue, except by continuing to
>>>>> allow CONFIG_HYPERV=m.
> 
> I'm wondering, is there a path for this change, then? Are there some
> intermediate step/s we could take to minimize the problem?
> 
>>>>
>>>> To certain extent, imo, users are expected to check config files
>>>> for changes when moving to new versions/releases, so it would be a
>>>> one time burden. 
>>>
>>> I'm not so sanguine about the impact. For those of us who work with
>>> Hyper-V frequently, yes, it's probably not that big of an issue -- we can
>>> figure it out. But a lot of Azure/Hyper-V users aren't that familiar with
>>> the details of how the Kconfig files are put together. And the issue occurs
>>> with no error messages that something has gone wrong in building
>>> the kernel, except that it won't boot. Just running "make olddefconfig"
>>> has worked in the past, so some users will be befuddled and end up
>>> generating Azure support incidents. I also wonder about breaking
>>> automated test suites for new kernels, as they are likely to be running
>>> "make olddefconfig" or something similar as part of the automation.
>>>
>>>> CONFIG_HYPERV=m is just broken imo as one sees that
>>>> in .config but magically symbols in drivers/hv are in kerenel.
>>>>
>>>
>>> I agree that's not ideal. But note that some Hyper-V code and symbols
>>> like ms_hyperv_init_platform() and related functions show up when
>>> CONFIG_HYPERVISOR_GUEST=y, even if CONFIG_HYPERV=n. That's
>>> the code in arch/x86/kernel/cpu/mshyperv.c and it's because Hyper-V
>>> is one of the recognized and somewhat hardwired hypervisors (like
>>> VMware, for example).
>>>
>>> Finally, there are about a dozen other places in the kernel that use
>>> the same Makefile construct to make some code built-in even though
>>> the CONFIG option is set to "m". That may not be enough occurrences
>>> to make it standard practice, but Hyper-V guests are certainly not the
>>> only case.
>>>
>>> In my mind, this is judgment call with no absolute right answer. What
>>> do others think about the tradeoffs?
>>
>> Wei had said in private message that he agrees this is a good idea. Nuno
>> said earlier above: 
>>
>> "FWIW I think it's a good idea, interested to hear what others think."
>>
> That was before Michael pointed out the potential issues which I was
> unaware of. Let's see if there's a path that is smoother for all the
> downstream users who may be compiling with CONFIG_HYPERV=m.

Ok, we've already thought of it for sometime and not able to come up
with any. IMO, it's a minor hickup, not major. This is stalling
upcoming iommu and other patches which will use CONFIG_HYPERV and 
add more dependencies, and it would be much harder to straighten 
out then. So I hope you guys can come up with some solution sooner than
later, I can't think of any.

Thanks,
-Mukesh

Powered by blists - more mailing lists