Re: [PATCH v9 10/12] vfio: Add function to get device_api string from vfio_device_info.flags

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

On Fri, 21 Oct 2016 01:59:55 +0530
Kirti Wankhede <kwankhede@nvidia.com> wrote:

> On 10/21/2016 1:04 AM, Alex Williamson wrote:
> > On Tue, 18 Oct 2016 02:52:10 +0530
> > Kirti Wankhede <kwankhede@nvidia.com> wrote:
> >   
> >> Function vfio_device_api_string() returns string based on flag set in
> >> vfio_device_info's flag. This should be used by vendor driver to get string
> >> based on flag for device_api attribute.
> >>
> >> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
> >> Signed-off-by: Neo Jia <cjia@nvidia.com>
> >> Change-Id: I42d29f475f02a7132ce13297fbf2b48f1da10995
> >> ---
> >>  drivers/vfio/vfio.c  | 15 +++++++++++++++
> >>  include/linux/vfio.h |  2 ++
> >>  2 files changed, 17 insertions(+)
> >>
> >> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> >> index 10ef1c5fa762..aec470454a13 100644
> >> --- a/drivers/vfio/vfio.c
> >> +++ b/drivers/vfio/vfio.c
> >> @@ -1917,6 +1917,21 @@ int vfio_set_irqs_validate_and_prepare(struct vfio_irq_set *hdr, int num_irqs,
> >>  }
> >>  EXPORT_SYMBOL(vfio_set_irqs_validate_and_prepare);
> >>  
> >> +const char *vfio_device_api_string(u32 flags)
> >> +{
> >> +	if (flags & VFIO_DEVICE_FLAGS_PCI)
> >> +		return "vfio-pci";
> >> +
> >> +	if (flags & VFIO_DEVICE_FLAGS_PLATFORM)
> >> +		return "vfio-platform";
> >> +
> >> +	if (flags & VFIO_DEVICE_FLAGS_AMBA)
> >> +		return "vfio-amba";
> >> +
> >> +	return "";
> >> +}
> >> +EXPORT_SYMBOL(vfio_device_api_string);
> >> +
> >>  /*
> >>   * Pin a set of guest PFNs and return their associated host PFNs for local
> >>   * domain only.
> >> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> >> index 31d059f1649b..fca2bf23c4f1 100644
> >> --- a/include/linux/vfio.h
> >> +++ b/include/linux/vfio.h
> >> @@ -116,6 +116,8 @@ extern int vfio_set_irqs_validate_and_prepare(struct vfio_irq_set *hdr,
> >>  					      int num_irqs, int max_irq_type,
> >>  					      size_t *data_size);
> >>  
> >> +extern const char *vfio_device_api_string(u32 flags);
> >> +
> >>  struct pci_dev;
> >>  #ifdef CONFIG_EEH
> >>  extern void vfio_spapr_pci_eeh_open(struct pci_dev *pdev);  
> > 
> > Couldn't this simply be a #define in the uapi header?
> > 
> > #define VFIO_DEVICE_PCI_API_STRING "vfio-pci"
> > 
> > I don't really see why we need a lookup function.
> >   
> 
> String is tightly coupled with the FLAG, right?
> Instead user need to take care of making sure to return proper string,
> and don't mis-match the string, I think having function is easier.

That's exactly why I proposed putting the #define string in the uapi,
by that I mean the vfio uapi header.  That keeps the tight coupling to
the flag, they're both defined in the same place, plus it gives
userspace a reference so they're not just inventing a string to compare
against.  IOW, the vendor driver simply does an sprintf of
VFIO_DEVICE_PCI_API_STRING and userspace (ie. libvirt) can do a strcmp
with VFIO_DEVICE_PCI_API_STRING from the same header and everybody
arrives at the same result.

> Vendor driver should decide the type of device they want to expose and
> set the flag, using this function vendor driver would return string
> which is based on flag they set.

Being a function adds no intrinsic value and being in a uapi header does
add value to userspace.  Thanks,

Alex