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 7F4E33C8717; Wed, 5 Aug 2026 17:24:11 +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=1785950652; cv=none; b=ksoODg1j7nt2OJgw7jXEpAgZHqKKHLAemJ8Bev5JXOmPk7cPETstFUPmS+ujlYn8iK80qcNP7KI/Wkv8G1LfRKeauRZwQXyJOEWKdHFaNr3R61s//Rr25Bmj+ppPt9HK+E5eRZZJJYzKLHkOB8A/L4yae+Q5VJTCmDITol5+V3A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785950652; c=relaxed/simple; bh=7Hra8MiiFA1bpttIEjZrJR+4edB5KH25Bj97EZLmDjo=; h=Message-ID:Date:MIME-Version:Cc:Subject:To:References:From: In-Reply-To:Content-Type; b=bpbMPgmfnVYYHO4Di+GGvNkVcoKD5iZjEn3sSjMzXo7rSxPw4u9H3fhVKKf4VS4nWlOPvQEkVUdp6G6GDOw0S8BhSUidV+9RqdmFDkKCWIZsKplJ1ySxrTR9pn2tBMmxJT+jIjevFgtkmVdxCrVZHWRPO+teqrDQ45aZl6IbH0E= 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=Dap7uAwL; 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="Dap7uAwL" Received: from [10.21.58.224] (unknown [52.172.102.253]) by linux.microsoft.com (Postfix) with ESMTPSA id 06A4320B7169; Wed, 5 Aug 2026 10:23:45 -0700 (PDT) DKIM-Filter: OpenDKIM Filter v2.11.0 linux.microsoft.com 06A4320B7169 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.microsoft.com; s=default; t=1785950630; bh=XPvRWFUhHMnNObZ6Hnjr7BBy003ESek6ZolrR6Wr9ek=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=Dap7uAwLIkg5aq1LJM38ldefrZkiYD9vaeCQHVIZh/yrU3ridtabIoDU3wSDGE+is 8mEGSgXsEqlf1P6Zo32jG5T3rrJ0Xfi3w4J34uWOKFRJ6be7ML+WjUQ0g1/XGY4z2s a5OKmV6MSnG8VTcs7RnhkD01wE0dmfGrJM1WijEs= Message-ID: Date: Wed, 5 Aug 2026 10:24:03 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Cc: easwar.hariharan@linux.microsoft.com, "kys@microsoft.com" , "haiyangz@microsoft.com" , "wei.liu@kernel.org" , "decui@microsoft.com" , "longli@microsoft.com" , "linux-hyperv@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition To: Michael Kelley References: <20260804190517.101981-1-mhklinux@outlook.com> From: Easwar Hariharan Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 8/4/2026 21:13, Michael Kelley wrote: > From: Easwar Hariharan Sent: Tuesday, August 4, 2026 3:49 PM >> >> On 8/4/2026 15:47, Easwar Hariharan wrote: >>> On 8/4/2026 15:17, Michael Kelley wrote: >>>> From: Easwar Hariharan Sent: Tuesday, August 4, 2026 12:40 PM >>>>> >>>>> On 8/4/2026 12:05, Michael Kelley wrote: >>>>>> The VMBus module should not be loaded when Linux is running directly >>>>>> in the root partition and root is not nested in another VM. Current >>>>>> code checks this condition and skips VMBus module initialization, which >>>>>> works. But it returns 0 as the result, so Linux thinks the module has >>>>>> successfully loaded. Later, if the module were to be unloaded, the >>>>>> VMBus module unload code tries to clean up things that were never >>>>>> initialized, resulting in memory faults and a panic. >>>>>> >>>>>> Fix this by having VMBus module initialization return -ENODEV for this >>>>>> case. The module is then not loaded, and the unload path can never run. >>>>>> >>>>>> Reported-by: Sashiko >>>>>> Closes: https://lore.kernel.org/linux- >>>>> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/ >>>>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root") >>>>>> Signed-off-by: Michael Kelley >>>>>> --- >>>>>> drivers/hv/vmbus_drv.c | 2 +- >>>>>> 1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c >>>>>> index e19ec73b0187..849d7e1a7320 100644 >>>>>> --- a/drivers/hv/vmbus_drv.c >>>>>> +++ b/drivers/hv/vmbus_drv.c >>>>>> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void) >>>>>> return -ENODEV; >>>>>> >>>>>> if (hv_root_partition() && !hv_nested) >>>>>> - return 0; >>>>>> + return -ENODEV; >>>>>> >>>>>> /* >>>>>> * Get ACPI resources first. >>>>> >>>>> This seems straightforward: >>>> >>>> Alas, it's not so straightforward, as Sashiko pointed out. I knew that >>>> the mshv module has a dependency on the vmbus module, but had >>>> forgotten. There's a reason for the dependency as described in the >>>> commit message for 840b740a35bf. >>>> >>>> There's another easy way to fix the VMBus module unload problem. >>>> I'll send a v2. :-) >>>> >>>> Michael >>>> >>> I may be missing something, but mshv_root is used in 3 cases: >>> >>> 1) Baremetal root partition, which requires no VMBus >>> >>> 2) L1VH aka Direct Virtualization, which is conditioned on hv_l1vh_partition(), which is >> not the check here in the vmbus driver >>> >>> 3) For the OpenHCL paravisor, where I honestly don't know what the dependency >> chain looks like, but based on a quick glance >>> at https://github.com/microsoft/OHCL-Linux-Kernel/ and a cursory grep, doesn't >> seem to rely on hv_root_partition() but does rely >>> on VMbus. >>> >>> I feel like Sashiko's review falls into item 1, but then again, there may just be a >> mismatch >>> between reality and my mental model, or my mental model may becorrect, but the >> code doesn't match it. >>> >>> Thanks, >>> Easwar (he/him) >> >> >> Well, number 4 is as a parent on a nested hypervisor, but !hv_nested takes care of that. >> > > For your #1, indeed the root partition does not require actual VMBus > functionality. But it *does* require that the VMBus module be loaded > if CONFIG_HYPERV_VMBUS=y. In that case, the only function the root > requires for #1 is the function hv_vmbus_exists(), and it requires that > to distinguish between your #1 and your #4 when it sets up the SynIC. > > If you want to build an image that only works for #1, then build with > CONFIG_HYPERV_VMBUS=n and there's no issue. But if you want an > image that works for #1 or #4, build with CONFIG_HYPERV_VMBUS=y > or =m, and the VMBus module must loaded so that hv_vmbus_exists() > gives the right answer (false for #1, true for #4). Furthermore, the > mshv_root dependency on the VMBus module ensures that the > VMBus module is always loaded before the mshv_root module so > that hv_vmbus_exists() will give an up-to-date answer when > mshv_root asks. > > The way the SynIC is managed should be refactored to better > handle #4, as Jork Loeser has proposed. Hopefully such a refactoring > would make the mshv_root->vmbus dependency go away. But until > such a refactoring is done, the dependency is what we have. > > I agree that your #2 and #3 are not relevant here. > > Michael Makes sense, I did not think of the distro user that'd probably like 1 image that serves all use cases. I'll look at your v2. Thanks, Easwar (he/him)