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

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Message-ID: <6687c068-33f0-c437-3106-bbd1d22f4d81@nvidia.com>
Date:   Fri, 21 Oct 2016 02:44:37 +0530
From:   Kirti Wankhede <kwankhede@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.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 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);

Kirti