Re: Memory allocation/sharing when DMA operations used..

[Bas Wijnen]

Hi,

I'm not very familiar with DMA, but I remembered that only certain pages
can be used for DMA.  Or was that only for ISA?  Anyway, if that is not
the case, it becomes a lot simpler.

The L4-Hurd design uses the idea to give the costs to the user (which is
a program in this context).  This is true in many places, in particular
for memory.  For example, if a client wants to read data from a
filesystem, it supplies a page to the filesystem, which then fills it
with data.

You suggest to leave that design for DMA.  If there are special
"DMA-pages", that doesn't sound strange: Otherwise a DoS-attack would be
possible by allocating many DMA pages.  Of course there could be a quota
system, which might be enough to prevent this.  If all pages can be used
for DMA, I don't see any reason not to use the normal approach of the
client providing the memory for the operation.

You say the user can die on you, leaving a big mess.  This is no
problem, as the container is shared by the driver and the user.  If the
user dies, the container is not deallocated until the driver lets go of
it.  It shouldn't do that before the DMA operation is finished.  It is
not unusual for device drivers to wait with freeing resources after
client death until the device has finished its operation.  This is not a
big problem.  I think it should not be a reason to leave a good design
decision.

If I misunderstood you, please rephrase.

Thanks,
Bas

Markus Kode Kaarn wrote:
> Hey everybody,
>
> For last few days i came whith idea of how Device-Drivers, should handle DMA
> requests. If DMA channels access can be considered as a priviledge of only
> PLM's, or even centralized DMA driver(which is prefered), through which will
> all DMA requests will be handled. First, i think DD's, that receives DMA
> request from user for the first time, should allocate/map memory for it selt
> by requesting it from physmem. Then share this region of memory whith
> task(as far as i can think these will be device drivers) that requests DMA.
> There would be read-only and the writeable memory shared, every of which
> is decided to be given whereas task want to receive(read) data, or
> send(write) it.
> Here i can see a good decision on that only a DMA driver should allocate and
> share memory regions whith tasks requesting the operations. Because allowing
> user to supply a buffer is not far clever, cause user can die or give up its
> pages back to physmem, and at time of receive-operation from DMA channel
> memory region could be used by some other task.
>
> This probably can apply not only to DMA operations. Some system part that
> provide service(s) to many tasks may can come whith this approach.
>
> At the moment i don't know much about the memory allocation/sharing in hurd-l4
> and can't be more specific on this, or supply code.
>
> Comments please.

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