Re: [Qemu-devel] [big lock] Discussion about the convention of device's DMA each other after breaking down biglock

[Avi Kivity <avi@redhat.com>]

On 09/24/2012 08:33 AM, liu ping fan wrote:
> On Wed, Sep 19, 2012 at 5:50 PM, Avi Kivity <avi@redhat.com> wrote:
> > On 09/19/2012 12:34 PM, Jan Kiszka wrote:
> >>
> >> What about the following:
> >>
> >> What we really need to support in practice is MMIO access triggers RAM
> >> access of device model. Scenarios where a device access triggers another
> >> MMIO access could likely just be rejected without causing troubles.
> >>
> >> So, when we dispatch a request to a device, we mark that the current
> >> thread is in a MMIO dispatch and reject any follow-up c_p_m_rw that does
> >> _not_ target RAM, ie. is another, nested MMIO request - independent of
> >> its destination. How much of the known issues would this solve? And what
> >> would remain open?
> >
> > Various iommu-like devices re-dispatch I/O, like changing endianness or
> > bitband.  I don't know whether it targets I/O rather than RAM.
> >
> Have not found the exact code. But I think the call chain may look
> like this: dev mmio-handler --> c_p_m_rw() --> iommu mmio-handler -->
> c_p_m_rw()
> And I think you worry about the case for "c_p_m_rw() --> iommu
> mmio-handler". Right? How about introduce an member can_nest for
> MemoryRegionOps of iommu's mr?
>

I would rather push the iommu logic into the memory API:

  memory_region_init_iommu(MemoryRegion *mr, const char *name,
                           MemoryRegion *target, MemoryRegionIOMMUOps *ops,
                           unsigned size)

  struct MemoryRegionIOMMUOps {
      target_physical_addr_t (*translate)(target_physical_addr_t addr,
bool write);
      void (*fault)(target_physical_addr_t addr);
  };

I'll look at a proposal for this.  It's a generalized case of
memory_region_init_alias().

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.