Re: Watchdog triggered while trying to access shared page

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Hi Ashish,

The overall model that you are trying to follow should work.
Just make sure that the EXIT function of producer-domU is not called
until the consuder-dom0 has finished reading the data.

More comments inline below, I found a couple of issues.


AshishKumar Mishra wrote:
> Hi All ,
> 
> 1) I am trying to learn Xen memory sharing by writing a piece of code to
> share a single page from  domU to dom0
>     I have an producer module running in domU sharing an page  & consumer
> module running in dom0
>     - Producer writes some string in page so that consumer get read the
> same string if the mapping is correct
> 
> 2) I am able to get the page mapped in consumer ( dom0)
>     But when i try to read the string written by producer ( domU) in
> consumer (dom0) ,
> * Xen watchdog is triggered *
>     Any input as to how to map a page between dom0 & domU ?
> 
> 3) The logic flow of producer & consumer is shared below   ( can share the
> code also if required )
> // ========= Page-Produmer-domU
> =======================================================
> *In INIT Function:     *
> 
>         frame = __get_free_pages(GFP_KERNEL, 1);
> 
>         framenum = virt_to_gfn(frame);
> 
> 
>         gref = gnttab_grant_foreign_access(DOM0_ID, framenum, 0);   //
> Grant access for one page
>         // Write some data to frame
> 
>         strcpy((char *)frame, "Data is written in frame \n");
> 
> 
> *In EXIT function : *
> 
>         gnttab_end_foreign_access(gref, 0, frame);
> 
> 
> 
> // ========= Page-Consumer-dom0
> =======================================================
> 
> *In INIT Function:*        struct gnttab_map_grant_ref ops;
> 
>         struct gnttab_unmap_grant_ref unmap_ops;
> 
> 
> 
>         typedef struct info_t {
> 
>                 grant_ref_t gref;
> 
>                 int domid;  /* remote domID */
> 
>         } info_t;
> 
>         info_t info;
> 
> 
> 
>         int gref;  // Value obtained from producer & passed as module-param
> 
>         int domid; // Domid of producer
> 
> 
> 
>         struct vm_struct *vm_start;
> 
>         info.gref = gref;
> 
>         info.domid = domid;
> 
> 
> 
>         vm_start = alloc_vm_area(PAGE_SIZE, NULL);
 
Can you try to use kmalloc, or better alloc_xenballooned_pages, instead
of alloc_vm_area to allocate the page? See:
drivers/xen/xenbus/xenbus_client.c:xenbus_map_ring_valloc_hvm as a
reference.


>         gnttab_set_map_op(&ops, (unsigned long)vm_start->addr,

You need to pass the guest physical address of the page/vm_area, not the
virtual address (i.e. the __pa() address).


> GNTMAP_host_map,info.gref, info.domid);
>         HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref, &ops, 1)
> 
>         // == This printk causes watchdog to be triggered
> =====================
>         printk("[Consumer-INIT]: We got string as = %s\n",
> (char*)vm_start->addr );
> 
> 
>         unmap_ops.host_addr = (unsigned long)(vm_start->addr);
> 
>         unmap_ops.handle = ops.handle;
 

 
 
> *In EXIT function : *
>     HYPERVISOR_grant_table_op(GNTTABOP_unmap_grant_ref, &unmap_ops, 1)
> 
> //
> ===============================================================================
> 
> Thanks
> Ashish