On 8/4/2026 15:47, Easwar Hariharan wrote: > On 8/4/2026 15:17, Michael Kelley wrote: >> From: Easwar Hariharan <[email protected]> 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 <[email protected]> >>>> Closes: https://lore.kernel.org/linux- >>> hyperv/[email protected]/ >>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if >>>> Linux is root") >>>> Signed-off-by: Michael Kelley <[email protected]> >>>> --- >>>> 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. - Easwar
