Re: [PATCH] drivers/hv: select PCI_HYPERV if PCI is enabled

[Wei Liu <wei.liu@kernel.org>]

On Mon, Jan 27, 2025 at 04:42:56PM -0500, Hamza Mahfooz wrote:
> On Mon, Jan 27, 2025 at 09:02:22PM +0000, Michael Kelley wrote:
> > From: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com> Sent: Monday, January 27, 2025 10:10 AM
> > > 
> > > We should select PCI_HYPERV here, otherwise it's possible for devices to
> > > not show up as expected, at least not in an orderly manner.
> > 
> > The commit message needs more precision:  What does "not show up"
> > mean, and what does "not in an orderly manner" mean?  And "it's possible"
> > is vague -- can you be more specific about the conditions?  Also, avoid
> > the use of personal pronouns like "we".
> > 
> > But the commit message notwithstanding, I don't think this is change
> > that should be made. CONFIG_PCI_HYPERV refers to the VMBus device
> > driver for handling vPCI (a.k.a PCI pass-thru) devices. It's perfectly
> > possible and normal for a VM on Hyper-V to not have any such devices,
> > in which case the driver isn't needed and should not be forced to be
> > included. (See Documentation/virt/hyperv/vpci.rst for more on vPCI
> > devices.)
> 
> Ya, we ran into an issue where CONFIG_NVME_CORE=y and
> CONFIG_PCI_HYPERV=m caused the passed-through SSDs not to show up (i.e.
> they aren't visible to userspace). I guess it's cause PCI_HYPERV has
> to load in before the nvme stuff for that workload. So, I thought it was
> reasonable to select PCI_HYPERV here to prevent someone else from
> shooting themselves in the foot. Though, I guess it really it on the
> distro guys to get that right.

Does inserting the PCI_HYPERV module trigger a (re)scanning of the
(v)PCI bus? If so, the passed-through NVMe devices should show up just
fine, I suppose.

I agree with Michael that we should not select PCI_HYPERV by default. In
some environments, it is not needed at all.

Thanks,
Wei.