Re: [PATCH v4] xen/iommu: cleanup iommu related domctl handling

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 22/04/2022 08:09, Jan Beulich wrote:
> On 21.04.2022 19:47, Andrew Cooper wrote:
>> On 20/04/2022 07:27, Jan Beulich wrote:
>>> On 20.04.2022 08:22, Juergen Gross wrote:
>>>> On 20.04.22 08:11, Jan Beulich wrote:
>>>>> On 20.04.2022 07:57, Juergen Gross wrote:
>>>>>> --- a/xen/include/xen/iommu.h
>>>>>> +++ b/xen/include/xen/iommu.h
>>>>>> @@ -341,8 +341,17 @@ struct domain_iommu {
>>>>>>   /* Does the IOMMU pagetable need to be kept synchronized with the P2M 
>>>>>> */
>>>>>>   #ifdef CONFIG_HAS_PASSTHROUGH
>>>>>>   #define need_iommu_pt_sync(d)     (dom_iommu(d)->need_sync)
>>>>>> +
>>>>>> +int iommu_do_domctl(struct xen_domctl *domctl, struct domain *d,
>>>>>> +                    XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl);
>>>>>>   #else
>>>>>>   #define need_iommu_pt_sync(d)     ({ (void)(d); false; })
>>>>>> +
>>>>>> +static inline int iommu_do_domctl(struct xen_domctl *domctl, struct 
>>>>>> domain *d,
>>>>>> +                                  XEN_GUEST_HANDLE_PARAM(xen_domctl_t) 
>>>>>> u_domctl)
>>>>>> +{
>>>>>> +    return -ENOSYS;
>>>>>> +}
>>>>> As said in reply to Andrew as well as in a number or earlier occasions,
>>>>> I firmly think that this wants to be -EOPNOTSUPP, not -ENOSYS. Views
>>>> In libxl there is an explicit check for ENOSYS being returned for
>>>> assigning/deassigning a device, same in the xc python bindings.
>>> Urgh.
>> Honestly, I wasn't particularly happy with your push to swap ENOSYS out
>> for EOPNOTSUPP.  This shows plainly why it's a bad move.
>>
>> An end user doesn't give two hoots about the distinction between
>> hypercall not supported and subops not supported; they care about
>> whether Xen can perform the requested action or not.  ENOSYS is the more
>> common way of signalling this, and having only one errno value to check
>> is better for everyone involved.
> End users are of little interest here. Code looking for ENOSYS is what
> is of interest, when the meaning of ENOSYS is quite well defined as
> "system call not implemented"

POSIX specifies it as "Functionality not supported" which is why other
compliment systems use it for smaller granularity than a system call.

~Andrew