Re: [Qemu-devel] [PATCH] dma: Define dma_context_memory and use in sysbus-ohci

[Peter Crosthwaite]

On Fri, Oct 26, 2012 at 10:48 AM, David Gibson
<address@hidden> wrote:
> On Thu, Oct 25, 2012 at 08:33:13PM +1000, Peter Crosthwaite wrote:
>> On Oct 24, 2012 3:27 AM, "Peter Maydell" <address@hidden> wrote:
>> >
>> > Define a new global dma_context_memory which is a DMAContext corresponding
>> > to the global address_space_memory AddressSpace. This can be used by
>> > sysbus peripherals like sysbus-ohci which need to do DMA.
>> >
>> > In particular, use it in the sysbus-ohci device, which fixes a
>> > segfault when attempting to use that device.
>> >
>> > Signed-off-by: Peter Maydell <address@hidden>
>> Reviewed-by: Peter Crosthwaite <address@hidden>
>
> Hrm.  So, as I originally conceived DMAContext, a NULL context pointer
> means "no translation" which is to say that DMA addresses are the same
> as memory space addresses.  Which would mean a context explicitly for
> this purpose should not be necessary.
>
> Has this assumption changed with the newer memory region integrated
> dma context stuff?


Yes, The Segfaulting line is in dma.h:

static inline int dma_memory_rw_relaxed(DMAContext *dma, dma_addr_t addr,
                                        void *buf, dma_addr_t len,
                                        DMADirection dir)
{
    if (!dma_has_iommu(dma)) {
        /* Fast-path for no IOMMU */
        address_space_rw(dma->as, addr, buf, len, dir ==
DMA_DIRECTION_FROM_DEVICE);
        return 0;
    } else {
        return iommu_dma_memory_rw(dma, addr, buf, len, dir);
    }
}

Dereferencing of dma->as segfaults sd dma==NULL in the cas you described.

>
> --
> David Gibson                    | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
>                                 | _way_ _around_!
> http://www.ozlabs.org/~dgibson
>