Re: [PATCH v7 2/5] sysctl: Add sysctl interface for querying PCI topology

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/24/2015 03:19 AM, Jan Beulich wrote:
>>>> On 24.04.15 at 00:20, <boris.ostrovsky@oracle.com> wrote:
>> On 04/21/2015 03:01 AM, Jan Beulich wrote:
>>>> +                 ((++dev_cnt > 0x3f) && hypercall_preempt_check()) )
>>>> +                break;
>>>> +        }
>>>> +
>>>> +        if ( (!ret || (ret == -ENODEV)) &&
>>>> +             __copy_field_to_guest(u_sysctl, op, u.pcitopoinfo.first_dev) )
>>>> +            ret = -EFAULT;
>>>> +    }
>>>> +    break;
>>>> +#endif
>>> With the continuation-less model now used I don't think it makes
>>> sense to have first_dev and num_devs - for re-invocation all the
>>> caller needs to do is increment the buffer pointer suitably. I.e.
>>> you can get away with just a count of devices afaict.
>>>
>> This would require walking xc_hypercall_buffer_t->hbuf. Would something like
>>
>>       set_xen_guest_handle_raw(sysctl..., (void *)HYPERCALL_BUFFER_AS_ARG(foo) + offset)
>>
>> be acceptable? I don't think I see anything better.
>>
>> I thought of adding set_xen_guest_handle_offset() that would look
>> similar to set_xen_guest_handle() but then I felt that having this in
>> API may not be a good idea since xc_hypercall_buffer_t->hbuf would end
>> up pointing to memory that is not allocated for full
>> xc_hypercall_buffer_t->sz.
> There ought to be a way to create a guest handle from other than
> the start of an allocated hypercall buffer, but that's really more a
> question for the tool stack folks.

Yes, this was question for toolstack people.

(And my second paragraph was not stated correctly, now that I re-read 
it. I meant to say that my understanding is that API is expected to make 
all safety checks on buffers and with set_xen_guest_handle_offset() that 
I was picturing in my head we could pass in pretty much any pointer. I 
suppose we could check 'hbuf+offset < sz')


-boris