linux-kernel - Re: [PATCH v9 10/12] vfio: Add function to get device_api string from vfio_device

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Message-ID: <20161020152220.57f6ee06@t450s.home>
Date:   Thu, 20 Oct 2016 15:22:20 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Kirti Wankhede <kwankhede@...dia.com>
Cc:     <pbonzini@...hat.com>, <kraxel@...hat.com>, <cjia@...dia.com>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <kevin.tian@...el.com>, <jike.song@...el.com>,
        <bjsdjshi@...ux.vnet.ibm.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 10/12] vfio: Add function to get device_api string
 from vfio_device_info.flags

On Fri, 21 Oct 2016 02:44:37 +0530
Kirti Wankhede <kwankhede@...dia.com> wrote:

> On 10/21/2016 2:35 AM, Alex Williamson wrote:
> > On Fri, 21 Oct 2016 01:59:55 +0530
> > Kirti Wankhede <kwankhede@...dia.com> wrote:
> >   
> >> On 10/21/2016 1:04 AM, Alex Williamson wrote:  
> >>> On Tue, 18 Oct 2016 02:52:10 +0530
> >>> Kirti Wankhede <kwankhede@...dia.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@...dia.com>
> >>>> Signed-off-by: Neo Jia <cjia@...dia.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,
> >   
> 
> Ok. The strings should be in uapi, but having function (like below) to
> return proper string based on flag would be good to have for vendor driver.
> 
>  +const char *vfio_device_api_string(u32 flags)
>  +{
>  +	if (flags & VFIO_DEVICE_FLAGS_PCI)
>  +		return VFIO_DEVICE_API_PCI_STRING;
>  +
>  +	if (flags & VFIO_DEVICE_FLAGS_PLATFORM)
>  +		return VFIO_DEVICE_API_PLATFORM_STRING;
>  +
>  +	if (flags & VFIO_DEVICE_FLAGS_AMBA)
>  +		return VFIO_DEVICE_API_AMBA_STRING;
>  +
>  +	return "";
>  +}
>  +EXPORT_SYMBOL(vfio_device_api_string);

I disagree, it's pointless maintenance overhead.  It's yet another
function that we need to care about for kABI and it offers almost no
value.  Thanks,

Alex