linux-kernel - Re: [PATCH] virtio: Change typecasts in vring

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Date:   Mon, 2 Sep 2019 09:56:42 +0000
From:   Matej Genci <matej.genci@...anix.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] virtio: Change typecasts in vring_init()

On 8/31/2019 6:43 PM, Michael S. Tsirkin wrote:
> On Fri, Aug 30, 2019 at 05:58:23PM +0000, Matej Genci wrote:
>> On 8/30/2019 3:02 PM, Michael S. Tsirkin wrote:
>>> On Tue, Aug 27, 2019 at 03:20:57PM +0000, Matej Genci wrote:
>>>> Compilers such as g++ 7.3 complain about assigning void* variable to
>>>> a non-void* variable (like struct pointers) and pointer arithmetics
>>>> on void*.
>>>>
>>>> Signed-off-by: Matej Genci <matej.genci@...anix.com>
>>>> ---
>>>>    include/uapi/linux/virtio_ring.h | 9 +++++----
>>>>    1 file changed, 5 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
>>>> index 4c4e24c291a5..2c339b9e2923 100644
>>>> --- a/include/uapi/linux/virtio_ring.h
>>>> +++ b/include/uapi/linux/virtio_ring.h
>>>> @@ -168,10 +168,11 @@ static inline void vring_init(struct vring *vr, unsigned int num, void *p,
>>>>    			      unsigned long align)
>>>>    {
>>>>    	vr->num = num;
>>>> -	vr->desc = p;
>>>> -	vr->avail = p + num*sizeof(struct vring_desc);
>>>> -	vr->used = (void *)(((uintptr_t)&vr->avail->ring[num] + sizeof(__virtio16)
>>>> -		+ align-1) & ~(align - 1));
>>>> +	vr->desc = (struct vring_desc *)p;
>>>> +	vr->avail = (struct vring_avail *)((uintptr_t)p
>>>> +		+ num*sizeof(struct vring_desc));
>>>> +	vr->used = (struct vring_used *)(((uintptr_t)&vr->avail->ring[num]
>>>> +		+ sizeof(__virtio16) + align-1) & ~(align - 1));
>>>>    }
>>>>    
>>>>    static inline unsigned vring_size(unsigned int num, unsigned long align)
>>>
>>> I'm not really interested in building with g++, sorry.
>>> Centainly not if it makes code less robust by forcing
>>> casts where they weren't previously necessary.
>>
>> Can you elaborate on how these casts make the code less robust?
> 
> If we ever change the variable types build will still pass
> because of the cast.
> 

Wouldn't that be the case in the original as well?
You're assigning void*, which is implicitly cast to everything.

>> They aren't necessary in C but I think being explicit can improve
>> readability as argued in
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__softwareengineering.stackexchange.com_a_275714&d=DwIBAg&c=s883GpUCOChKOHiocYtGcg&r=dbPDDn52JgZndd-WPvGcL5PLZTrms-72TItYJx-If5I&m=sw6xxC2EOF9g3XtUKuI6OvT5xhYF7XcWBqyQvGb-UMw&s=QWoZHF4XlOzPesnnbfsf1_KORrzkXb6yfd6yQGCwepc&e=
>>
>>>
>>> However, vring_init and vring_size are legacy APIs anyway,
>>> so I'd be happy to add ifndefs that will allow userspace
>>> simply hide these functions if it does not need them.
>>>
>>
>> I feel like my patch is a harmless way of allowing this header
>> to be used in C++ projects, but I'm happy to drop it in lieu of
>> the guards if you feel strongly about it.
>>
>> Thanks.
>> Matej
> 
> Yea let's not even start.
> 

Sure. I can re-send the patch with guards. But for my own sake,
can you elaborate on the above?

>>>
>>>> -- 
>>>> 2.22.0
>>>>
>>