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

Pierre Morel
2019-05-10T15:36:05Z

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.
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.
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