Re: [PATCH] vfio/pci: make MSI handling optional

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2014-10-09 at 10:40 +0200, Arnd Bergmann wrote:
> A recent bug fix to the MSIx handling in vfio added references
> to functions that may not be defined if MSI is disabled in the kernel,
> resulting in this link error:
> 
> drivers/built-in.o: In function `vfio_msi_set_vector_signal':
> :(.text+0x450808): undefined reference to `get_cached_msi_msg'
> :(.text+0x45080c): undefined reference to `write_msi_msg'
> 
> From what I can tell, all the MSI specific ioctl handling can
> be made optional in this case, which also reduces the code size
> for the vfo driver. This patches does so using a build-time
> IS_ENABLED() check, which leaves all the build coverage testing
> present but avoids the link error.
> 
> A side-effect of this is that the driver now returns -ENOTTY instead
> of -EINVAL if user space calls VFIO_PCI_MSI_IRQ_INDEX on a kernel
> without MSI support, which seems to be the best behavior, but
> the approach could easily be changed if we want to preserve the
> existing behavior for compatibility reasons.
> 
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Fixes: b8f02af096b1 ("vfio/pci: Restore MSIx message prior to enabling")
> ---

Why wouldn't we handle this with stubs for `get_cached_msi_msg' and
`write_msi_msg'?  We're really not talking about much code that might
get removed by the compiler with this static branch and and it seems
like a rather non-standard mechanism.  The count of MSI/X IRQs
advertised to the user should be zero without CONFIG_MSI and later range
checks would prevent calls to
these functions for invalid indexes, so it's a bit of a random test in
the code flow.  Thanks,

Alex

> Found using randconfig build testing on ARM.
> 
> diff --git a/drivers/vfio/pci/vfio_pci_intrs.c b/drivers/vfio/pci/vfio_pci_intrs.c
> index 553212f037c3..1117f96b8556 100644
> --- a/drivers/vfio/pci/vfio_pci_intrs.c
> +++ b/drivers/vfio/pci/vfio_pci_intrs.c
> @@ -827,6 +827,9 @@ int vfio_pci_set_irqs_ioctl(struct vfio_pci_device *vdev, uint32_t flags,
>  		break;
>  	case VFIO_PCI_MSI_IRQ_INDEX:
>  	case VFIO_PCI_MSIX_IRQ_INDEX:
> +		if (!IS_ENABLED(CONFIG_PCI_MSI))
> +			break;
> +
>  		switch (flags & VFIO_IRQ_SET_ACTION_TYPE_MASK) {
>  		case VFIO_IRQ_SET_ACTION_MASK:
>  		case VFIO_IRQ_SET_ACTION_UNMASK:
>