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

On 26.01.21 02:14, Oleksandr wrote:

Hello, all

On 26.01.21 01:20, Julien Grall wrote:

Hi Julien, Stefano

On Mon, 25 Jan 2021 at 20:56, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:

This seems to be an arm randconfig failure:

Thanks! The error is:

#'target_mem_ref' not supported by expression#'memory.c: In function
memory.c:1210:18: error:  may be used uninitialized in this function
  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?
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...


Oleksandr Tyshchenko



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