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 5498E2FFDC6; Thu, 4 Sep 2025 18:18:44 +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=1757009926; cv=none; b=PqNC4WDQhcr5X4cTaLQapQN3f0wfZch5ZHIBGdXEf65EsCgrAFPlX1ho0TrPGH3tPW0DV48NQSRtacoziAOuONNR8cBvF0xEqOjQh9R3MAWauYqx/LaPGo04SR5sOywgSbYMuMMO5D1fzGdBxY6LHHF/KPlFWPjXEiT2vHi676w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757009926; c=relaxed/simple; bh=7c8nj6KxULONPPQj8MkFkKHnBlddpgD0kXlRJfUjxGA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=sqD79JzV802fdS+RCelu9vaq6qx8m0lyWVf+88uTEQPhpD3XyWzH1odZhlEvERvLKTJcwzNQzMAypYisWzE13dL84xYOMztZLl2K9OduRUKx/wBjnKV0KJ7r7f55EEx2d1PWGQARFRH++RAANZREtTSzA3Yx6pkxWBFQzSSuIEo= 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=oLu6V+pW; 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="oLu6V+pW" Received: from [192.168.0.88] (192-184-212-33.fiber.dynamic.sonic.net [192.184.212.33]) by linux.microsoft.com (Postfix) with ESMTPSA id 6A3772119CB7; Thu, 4 Sep 2025 11:18:42 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 6A3772119CB7 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1757009923; bh=d/D6Qrj0sP6g8qAnJASJd8z3FHjIIoC1QYzFJbAmmgM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=oLu6V+pWgKTtTfQLhdxwHfvtkUJ0N0iCi/cfaSJbTLxtH/SZHgcXWgxsK9ZoNkfuI Jhe+3szLxA45hPC786Cup2SrThBmJ//Jy32lAGJDJ0uCT8yGVHH2WlO1x+gdZ2XUBb VnhKgbbsVrL7mHJ17eqOI/RKV3FeYxFdGYLhbtKQ= Message-ID: <4f38c613-255c-eaf6-0d50-28f8ffc02fff@linux.microsoft.com> Date: Thu, 4 Sep 2025 11:18:41 -0700 Precedence: bulk X-Mailing-List: linux-fbdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.1 Subject: Re: [PATCH V0 0/2] Fix CONFIG_HYPERV and vmbus related anamoly Content-Language: en-US To: 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> From: Mukesh R In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. >>> >>> 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. >> >> 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." Thanks, -Mukesh