Re: [Qemu-devel] [RFC] Memory API

[Gleb Natapov]

On Thu, May 19, 2011 at 03:37:58PM +0200, Jan Kiszka wrote:
> On 2011-05-19 15:36, Anthony Liguori wrote:
> > On 05/18/2011 02:40 PM, Jan Kiszka wrote:
> >> On 2011-05-18 15:12, Avi Kivity wrote:
> >>> void cpu_register_memory_region(MemoryRegion *mr, target_phys_addr_t
> >>> addr);
> >>
> >> OK, let's allow overlapping, but make it explicit:
> >>
> >> void cpu_register_memory_region_overlap(MemoryRegion *mr,
> >>                                          target_phys_addr_t addr,
> >>                                          int priority);
> > 
> > The device doesn't actually know how overlapping is handled.  This is
> > based on the bus hierarchy.
> 
> Devices don't register their regions, buses do.
> 
Today PCI device may register region that overlaps with any other
registered memory region without even knowing it. Guest can write any
RAM address into PCI BAR and this RAM address will be come mmio are. More
interesting is what happens when guest reprogram PCI BAR to other address
- the RAM that was at the previous address just disappears. Obviously
  this is crazy behaviour, but the question is how do we want to handle
it? One option is to disallow such overlapping registration, another is
to restore RAM mapping after PCI BAR is reprogrammed. If we chose second
one the PCI will not know that _overlap() should be called.

Another example may be APIC region and PCI. They overlap, but neither
CPU nor PCI knows about it.

--
                        Gleb.