Flexpage: shared Mem

ba_f ba_f at rbg.informatik.tu-darmstadt.de
Thu May 12 16:09:47 CEST 2016


Am 2016-05-12 00:20, schrieb Adam Lackorzynski:
> Hi,
>> /* client.c */
>>   l4_fpage_t fpage = l4_obj_fpage( dataspace, 0, L4_CAP_FPAGE_RW );
>>   l4_utcb_mr()->mr[0] = size;
>>   l4_utcb_br()->br[0] = fpage.raw;
> That is more like a setup for a receive. Please have a look at some
> calls in l4sys, such as l4_scheduler_run_thread or l4_task_map which
> send caps to the callee. Note that the l4_map_obj_control+l4_obj_fpage
> pair(s) must be at the end of the MR-array. And do not forget proper
> values for the sending l4_msgtag.

This is what my trial(!) looks now:

/* client.c */

   l4_utcb_mr()->mr[0] = size;
   l4_utcb_mr()->mr[1] = l4_map_obj_control(dataspace, L4_ITEM_CONT);
   l4_utcb_mr()->mr[2] = l4_obj_fpage( dataspace, 0, L4_CAP_FPAGE_RW 

/* server.c */

   l4_utcb_br()->bdr = 0;
//  l4_utcb_br()->br[0] = l4_map_obj_control(dataspace, 
   l4_utcb_br()->br[0] = L4_ITEM_MAP;
   l4_utcb_br()->br[1] = l4_obj_fpage(dataspace, 0, L4_CAP_FPAGE_RW).raw;

   tag = l4_ipc_wait(l4_utcb(), &label, L4_IPC_NEVER);
     r = l4_ipc_error(tag, l4_utcb());
     if ( r) {
       fprintf(stderr, "IPC error: %x\n", r);
       tag = l4_ipc_wait(l4_utcb(), &label, L4_IPC_NEVER);
     tag = l4_ipc_reply_and_wait(l4_utcb(), l4_msgtag(0, 1, 0, 0),
				&label, L4_IPC_NEVER);

This gives me "IPC error: 9" when doing l4_ipc_reply_and_wait().
I don't receive a dataspace from client, neither.

>> Anyway, maybe I need some more understandings about capabilities &
>> flexpages.
>> (Are there any precise slides or papers about this topic?)
>> So, the client creates a dataspace capability and allocates some 
>> memory.
>> But, what happens when the flexpage is pushed to the server?
>> The capability is copied and both pointing to the same allocated 
>> memory, or
>> is the Cap and(!) the memory it is pointing to copied to server?
> When you transfer a capability to a dataspace, you just transfer the
> access rights to that particular dataspace to the other party. Not the
> memory. After the transfer, both can talk to the very same dataspace,
> map it and use it. Typically when sharing dataspaces that is called
> shared memory.
> Adam

Actually, I would like to understand what's going on.
I know every thread has its UTCBs and an IPC_CALL just copies the MRs 
from caller to callee.
But, flexpages seem to be a special case.
I guess, the BRs are for receiving flexpages. And there must be some 
kind of parsing of the MRs since l4_map_obj_control+l4_obj_fpage pair(s) 
are recognized.
And somehow the kernel must find the matching BRs.
Where does all this stuff happen?



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