From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from linux.microsoft.com (linux.microsoft.com [13.77.154.182]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 34184280A5F; Fri, 5 Sep 2025 20:08:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=13.77.154.182 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757102884; cv=none; b=esA16IQtJunG5a/yJlCxqpuHOTonFUMzxg8Cho092RblnRaQJs+A/xWdfhapuaemdUf6VJB3grEGd5/QmCZ8LHFMR+hYxF7ckA4XSXnQA7t2cyqTiAZgZLvfvxBeYby7n9ctEb1AGOi3Vio3OYLNdllXTe4KK0IixudKBpblwWE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757102884; c=relaxed/simple; bh=2m6fPzkCRC92qTFZavMPj/0W9huhFTFWzZEWQiM0e+Y=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=XHkWP3iDOcjvFbJHx4muxFHr7g4fXfk81u3/MHI+YSg72AqwIglilC0FypKhl+1OanhWhH+U1qG/f8bzr7/WRGgS4cfuZKeNrjkQvqDgsSsE8bZqa08UVVeWVgeK443OLjG8LY89tUgG3QaCLNZlLMJsPE6TqsvI9oI9XolngIY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com; spf=pass smtp.mailfrom=linux.microsoft.com; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b=bsZ+Tv9v; arc=none smtp.client-ip=13.77.154.182 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.microsoft.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.microsoft.com header.i=@linux.microsoft.com header.b="bsZ+Tv9v" Received: from [10.0.0.114] (c-67-182-156-199.hsd1.wa.comcast.net [67.182.156.199]) by linux.microsoft.com (Postfix) with ESMTPSA id 98B5220171C4; Fri, 5 Sep 2025 13:08:01 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 98B5220171C4 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1757102882; bh=uotTTD65zan8TmCm5oKAbPiE7QB78kRP80SKbUy3768=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=bsZ+Tv9vlFvx3FSsyyJQ00ypVCg071rBzhVBQzQ/LnPzkj/syqiM8BQWw8v4Czew8 +mS7K/hwRDMDO3GD3r5fPtl8h2fOvZArSQGq0dxtTszR8YTrcw26eytE3rGQ7MMIqk vnwEKn2ou0NMnA2ymmTpNmSgop+6sZ4I9ED+9giU= Message-ID: <231f05cb-4f33-48ac-bb2e-1359ed52e606@linux.microsoft.com> Date: Fri, 5 Sep 2025 13:08:00 -0700 Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly To: Mukesh R , Michael Kelley , "dri-devel@lists.freedesktop.org" , "linux-kernel@vger.kernel.org" , "linux-input@vger.kernel.org" , "linux-hyperv@vger.kernel.org" , "netdev@vger.kernel.org" , "linux-pci@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fbdev@vger.kernel.org" , "linux-arch@vger.kernel.org" , "virtualization@lists.linux.dev" Cc: "maarten.lankhorst@linux.intel.com" , "mripard@kernel.org" , "tzimmermann@suse.de" , "airlied@gmail.com" , "simona@ffwll.ch" , "jikos@kernel.org" , "bentiss@kernel.org" , "kys@microsoft.com" , "haiyangz@microsoft.com" , "wei.liu@kernel.org" , "decui@microsoft.com" , "dmitry.torokhov@gmail.com" , "andrew+netdev@lunn.ch" , "davem@davemloft.net" , "edumazet@google.com" , "kuba@kernel.org" , "pabeni@redhat.com" , "bhelgaas@google.com" , "James.Bottomley@HansenPartnership.com" , "martin.petersen@oracle.com" , "gregkh@linuxfoundation.org" , "deller@gmx.de" , "arnd@arndb.de" , "sgarzare@redhat.com" , "horms@kernel.org" References: <20250828005952.884343-1-mrathor@linux.microsoft.com> <4f38c613-255c-eaf6-0d50-28f8ffc02fff@linux.microsoft.com> Content-Language: en-US From: Nuno Das Neves In-Reply-To: <4f38c613-255c-eaf6-0d50-28f8ffc02fff@linux.microsoft.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/4/2025 11:18 AM, Mukesh R wrote: > On 9/4/25 09:26, Michael Kelley wrote: >> From: Mukesh R Sent: Wednesday, September 3, 2025 7:17 PM >>> >>> On 9/2/25 07:42, Michael Kelley wrote: >>>> From: Mukesh Rathor 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. Nuno > Thanks,> -Mukesh > >