Re: [PATCH V5 00/22] IOREQ feature (+ virtio-mmio) on Arm

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.01.2021 12:15, Oleksandr wrote:
> On 27.01.21 12:51, Jan Beulich wrote:
>> On 27.01.2021 11:13, Oleksandr wrote:
>>> On 26.01.21 02:14, Oleksandr wrote:
>>>> On 26.01.21 01:20, Julien Grall wrote:
>>>>> On Mon, 25 Jan 2021 at 20:56, Stefano Stabellini
>>>>> <sstabellini@xxxxxxxxxx> wrote:
>>>>>> This seems to be an arm randconfig failure:
>>>>>>
>>>>>> https://gitlab.com/xen-project/patchew/xen/-/pipelines/246632953
>>>>>> https://gitlab.com/xen-project/patchew/xen/-/jobs/985455044
>>>>> Thanks! The error is:
>>>>>
>>>>> #'target_mem_ref' not supported by expression#'memory.c: In function
>> Btw, I found the first part of this line pretty confusing, to a
>> degree that when seeing it initially I thought this must be some
>> odd tool producing the odd error. But perhaps this is just
>> unfortunate output ordering from different tools running in
>> parallel.
>>
>>>>> 'do_memory_op':
>>>>> memory.c:1210:18: error:  may be used uninitialized in this function
>>>>> [-Werror=maybe-uninitialized]
>>>>>    1210 |             rc = set_foreign_p2m_entry(currd, d, gfn_list[i],
>>>>>         | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>    1211 | _mfn(mfn_list[i]));
>>>>>         | ~~~~~~~~~~~~~~~~~~
>>>>>
>>>>> I found a few references online of the error message, but it is not
>>>>> clear what it means. From a quick look at Oleksandr's branch, I also
>>>>> can't spot anything unitialized. Any ideas?
>>>> It seems that error happens if *both* CONFIG_GRANT_TABLE and
>>>> CONFIG_IOREQ_SERVER are disabled. Looks like that mfn_list is
>>>> initialized either in acquire_grant_table() or in acquire_ioreq_server().
>>>> If these options disabled then corresponding helpers are just stubs,
>>>> so indeed that mfn_list gets uninitialized. But, I am not sure why gcc
>>>> complains about it as set_foreign_p2m_entry() is *not* going to be
>>>> called in that case???
>>> This weird build error goes away if I simply add:
>>>
>>> diff --git a/xen/common/memory.c b/xen/common/memory.c
>>> index 33296e6..d1bd57b 100644
>>> --- a/xen/common/memory.c
>>> +++ b/xen/common/memory.c
>>> @@ -1136,7 +1136,7 @@ static int acquire_resource(
>>>         * moment since they are small, but if they need to grow in future
>>>         * use-cases then per-CPU arrays or heap allocations may be required.
>>>         */
>>> -    xen_pfn_t mfn_list[32];
>>> +    xen_pfn_t mfn_list[32] = {0};
>>>        int rc;
>>>
>>>        if ( !arch_acquire_resource_check(currd) )
>>>
>>>
>>> Shall I make the corresponding patch?
>> I'd prefer if we could find another solution, avoiding this
>> pointless writing of 256 bytes of zeros (and really to be on the
>> safe side I think it should rather be ~0 that gets put in there).
>> Could you check whether clearing the array along the lines of
>> this
>>
>>      default:
>>          memset(mfn_list, ~0, sizeof(mfn_list));
>>          rc = -EOPNOTSUPP;
>>          break;
>>
>> helps (avoiding the writes in all normal cases)?
> 
> Yes, this helps (at least in my environment):
> 
> aarch64-poky-linux-gcc v8.2

Good. I'd be okay if you folded this in (plus a comment of
course), but others may have different views, not the least as
this is only papering over the issue (yet an issue that's not
ours, but the compiler's).

Jan