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 837102F83AF; Fri, 5 Sep 2025 21:41:58 +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=1757108520; cv=none; b=MQJqL/pGz2CFe2fkZl2KyT1aejkWRldRcNFE+8xoZ3zWm1i48uR7NWuUXgw0qFdnX7h9JBM75j0I9yf9Lq2m1mFTqeTeOs/z28I50wmnOqPnOviCO47QnkTP8X4OCWGMfd+Uz0J8saOBYMgUPNNXwPyWZiUxsPGFq2Ho6OsVRaM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757108520; c=relaxed/simple; bh=n1zL1dghlRy0+A8PIhiVW/ZsMjbvNqyCZS6VPJicZms=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ULjqqPZZB6CNffRIHL9Z1SeaR67XtwP2lg2+kcfxU7Nyc4F30/DxK6V/2O+1C8z8kaPttbr7PT+7Du6Rh6RgA4SDDzAXYoDwnsrBtzOmW2iWhvSepkNDvhdWuyASaLfoGav0xNT6xEvX7ApAn0a7ZixX3aXj/A5eIB3+OMOwKp8= 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=RBETcqgZ; 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="RBETcqgZ" 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 7E61E20171D3; Fri, 5 Sep 2025 14:41:53 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 7E61E20171D3 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1757108514; bh=pLTx4Rr8G6NQmZqkGo3pXfMpnw5bZX2ma0h+xMq8JrA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=RBETcqgZyxhP4vA+SY74JeEifpJbs8CGGsV8LNL4b3jFuP9zvIq+hI8kC5wZPgcC6 1WT2cgW4wYPCxkj6xOu4yy4jAknNRtnBrBlRHaMKitBQ2PX1olNc+ZlDBCF0VUFtnh NIUJ2ivqX8aQgh1dtNqaU2+PbzVKxt32fdpclyYk= Message-ID: <6a26cbf8-7877-4f39-0ed3-7bbc306f9fe5@linux.microsoft.com> Date: Fri, 5 Sep 2025 14:41:52 -0700 Precedence: bulk X-Mailing-List: linux-hyperv@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: Nuno Das Neves , 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> <231f05cb-4f33-48ac-bb2e-1359ed52e606@linux.microsoft.com> From: Mukesh R In-Reply-To: <231f05cb-4f33-48ac-bb2e-1359ed52e606@linux.microsoft.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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 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. 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