Re: [PATCH 08/10] virtio/s390: add indirection to indicators access

[Pierre Morel <pmorel@linux.ibm.com>]

On 13/05/2019 12:15, Cornelia Huck wrote:
> On Fri, 10 May 2019 17:36:05 +0200
> Pierre Morel <pmorel@linux.ibm.com> wrote:
> 
>> On 10/05/2019 13:54, Halil Pasic wrote:
>>> On Fri, 10 May 2019 09:43:08 +0200
>>> Pierre Morel <pmorel@linux.ibm.com> wrote:
>>>    
>>>> On 09/05/2019 20:26, Halil Pasic wrote:
>>>>> On Thu, 9 May 2019 14:01:01 +0200
>>>>> Pierre Morel <pmorel@linux.ibm.com> wrote:
>>>>>   
>>>>>> On 08/05/2019 16:31, Pierre Morel wrote:
>>>>>>> On 26/04/2019 20:32, Halil Pasic wrote:
>>>>>>>> This will come in handy soon when we pull out the indicators from
>>>>>>>> virtio_ccw_device to a memory area that is shared with the hypervisor
>>>>>>>> (in particular for protected virtualization guests).
>>>>>>>>
>>>>>>>> Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
>>>>>>>> ---
>>>>>>>>      drivers/s390/virtio/virtio_ccw.c | 40
>>>>>>>> +++++++++++++++++++++++++---------------
>>>>>>>>      1 file changed, 25 insertions(+), 15 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/s390/virtio/virtio_ccw.c
>>>>>>>> b/drivers/s390/virtio/virtio_ccw.c
>>>>>>>> index bb7a92316fc8..1f3e7d56924f 100644
>>>>>>>> --- a/drivers/s390/virtio/virtio_ccw.c
>>>>>>>> +++ b/drivers/s390/virtio/virtio_ccw.c
>>>>>>>> @@ -68,6 +68,16 @@ struct virtio_ccw_device {
>>>>>>>>          void *airq_info;
>>>>>>>>      };
>>>>>>>> +static inline unsigned long *indicators(struct virtio_ccw_device *vcdev)
>>>>>>>> +{
>>>>>>>> +    return &vcdev->indicators;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>> +static inline unsigned long *indicators2(struct virtio_ccw_device
>>>>>>>> *vcdev)
>>>>>>>> +{
>>>>>>>> +    return &vcdev->indicators2;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>      struct vq_info_block_legacy {
>>>>>>>>          __u64 queue;
>>>>>>>>          __u32 align;
>>>>>>>> @@ -337,17 +347,17 @@ static void virtio_ccw_drop_indicator(struct
>>>>>>>> virtio_ccw_device *vcdev,
>>>>>>>>              ccw->cda = (__u32)(unsigned long) thinint_area;
>>>>>>>>          } else {
>>>>>>>>              /* payload is the address of the indicators */
>>>>>>>> -        indicatorp = kmalloc(sizeof(&vcdev->indicators),
>>>>>>>> +        indicatorp = kmalloc(sizeof(indicators(vcdev)),
>>>>>>>>                           GFP_DMA | GFP_KERNEL);
>>>>>>>>              if (!indicatorp)
>>>>>>>>                  return;
>>>>>>>>              *indicatorp = 0;
>>>>>>>>              ccw->cmd_code = CCW_CMD_SET_IND;
>>>>>>>> -        ccw->count = sizeof(&vcdev->indicators);
>>>>>>>> +        ccw->count = sizeof(indicators(vcdev));
>>>>>>>
>>>>>>> This looks strange to me. Was already weird before.
>>>>>>> Lucky we are indicators are long...
>>>>>>> may be just sizeof(long)
>>>>>>   
>>>>>
>>>>> I'm not sure I understand where are you coming from...
>>>>>
>>>>> With CCW_CMD_SET_IND we tell the hypervisor the guest physical address
>>>>> at which the so called classic indicators. There is a comment that
>>>>> makes this obvious. The argument of the sizeof was and remained a
>>>>> pointer type. AFAIU this is what bothers you.
>>>>>>
>>>>>> AFAIK the size of the indicators (AIV/AIS) is not restricted by the
>>>>>> architecture.
>>>>>
>>>>> The size of vcdev->indicators is restricted or defined by the virtio
>>>>> specification. Please have a look at '4.3.2.6.1 Setting Up Classic Queue
>>>>> Indicators' here:
>>>>> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-1630002
>>>>>
>>>>> Since with Linux on s390 only 64 bit is supported, both the sizes are in
>>>>> line with the specification. Using u64 would semantically match the spec
>>>>> better, modulo pre virtio 1.0 which ain't specified. I did not want to
>>>>> do changes that are not necessary for what I'm trying to accomplish. If
>>>>> we want we can change these to u64 with a patch on top.
>>>>
>>>> I mean you are changing these line already, so why not doing it right
>>>> while at it?
>>>>   
>>>
>>> This patch is about adding the indirection so we can move the member
>>> painlessly. Mixing in different stuff would be a bad practice.
>>>
>>> BTW I just explained that it ain't wrong, so I really do not understand
>>> what do you mean by  'why not doing it right'. Can you please explain?
>>>    
>>
>> I did not wanted to discuss a long time on this and gave my R-B, so
>> meaning that I am OK with this patch.
>>
>> But if you ask, yes I can, it seems quite obvious.
>> When you build a CCW you give the pointer to CCW->cda and you give the
>> size of the transfer in CCW->count.
>>
>> Here the count is initialized with the sizeof of the pointer used to
>> initialize CCW->cda with.
> 
> But the cda points to the pointer address, so the size of the pointer
> is actually the correct value here, isn't it?

Oh. Yes, it is correct.
What I do not like are the mixing of (unsigned long), (unsigned long *) 
and &
if we had
cda = _u32 (unsigned long) indicatorp
count = sizeof(*indicatorp)

I would have been completely happy.

It was just a non important thing and I wouldn't have given a R-B if the 
functionality was not correct.


> 
>> Lukily we work on a 64 bits machine with 64 bits pointers and the size
>> of the pointed object is 64 bits wide so... the resulting count is right.
>> But it is not the correct way to do it.
> 
> I think it is, but this interface really is confusing.

Yes, it is what I thought we could do better.

> 
>> That is all. Not a big concern, you do not need to change it, as you
>> said it can be done in another patch.
>>
>>> Did you agree with the rest of my comment? I mean there was more to it.
>>>    
>>
>> I understood from your comments that the indicators in Linux are 64bits
>> wide so all OK.
>>
>> Regards
>> Pierre
>>
>>
>>
>>
>>
>>
> 


-- 
Pierre Morel
Linux/KVM/QEMU in Böblingen - Germany