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

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Date:   Fri, 30 Aug 2019 17:58:23 +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 8/30/2019 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?
They aren't necessary in C but I think being explicit can improve
readability as argued in
https://softwareengineering.stackexchange.com/a/275714

> 
> 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

> 
>> -- 
>> 2.22.0
>>