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

[Jan Beulich <jbeulich@xxxxxxxx>]

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)? Of course this
wouldn't be a guarantee that another compiler (version) won't
warn yet again. But the only other alternative I can think of
without having the writes on the common path would be something
along the lines of older Linux'es uninitialized_var(). Maybe
someone else has a better idea ...

> But it is still unclear to me why the compiler doesn't recognize that 
> *non-yet-uninitialized* mfn_list[] won't be used if both 
> CONFIG_GRANT_TABLE and CONFIG_IOREQ_SERVER are not set...

The combination of conditions may be too complex for it to
figure. I suppose a warning like this can't be had without at
least one of false positives or false negatives.

Jan