Re: [Qemu-devel] [RFC v1] Add declarations for hierarchical memory region API

[Avi Kivity <avi@redhat.com>]

On 05/22/2011 12:32 PM, Blue Swirl wrote:
> >>  >    +void memory_region_add_coalescing(MemoryRegion *mr,
> >>  >    +                                  target_phys_addr_t offset,
> >>  >    +                                  target_phys_addr_t size);
> >>  >    +/* Disable MMIO coalescing for the region. */
> >>  >    +void memory_region_clear_coalescing(MemoryRegion *mr);
> >>
> >>  Perhaps the interface could be more generic, like
> >>  +void memory_region_set_property(MemoryRegion *mr, unsigned flags);
> >>  +void memory_region_clear_property(MemoryRegion *mr, unsigned flags);
> >>
> >
> >  Coalescing is a complex property, not just a boolean attribute.  We probably
> >  will have a number of boolean attributes later, though.
>
> But what is the difference between adding coalescing to an area and
> setting the bit property 'coalescing' to an area? At least what you
> propose now is not so complex that it couldn't be handled as a single
> bit.

Look at the API - add_coalescing() sets the coalescing property on a 
subrange of the memory region, not the entire region.

(motivation - hw/e1000.c).

> >>  >    + * conflicts are resolved by having a higher @priority hide a lower
> >>  >  @priority.
> >>  >    + * Subregions without priority are taken as @priority 0.
> >>  >    + */
> >>  >    +void memory_region_add_subregion_overlap(MemoryRegion *mr,
> >>  >    +                                         target_phys_addr_t offset,
> >>  >    +                                         MemoryRegion *subregion,
> >>  >    +                                         unsigned priority);
> >>  >    +/* Remove a subregion. */
> >>  >    +void memory_region_del_subregion(MemoryRegion *mr,
> >>  >    +                                 MemoryRegion *subregion);
> >>
> >>  What would the subregions be used for?
> >
> >  Subregions describe the flow of data through the memory bus.  We'd have a
> >  subregion for the PCI bus, with its own subregions for various BARs, with
> >  some having subregions for dispatching different MMIO types within the BAR.
> >
> >  This allows, for example, the PCI layer to move a BAR without the PCI device
> >  knowing anything about it.
>
> But why can't a first class region be used for that?

Subregions are first-class regions.  In fact all regions are subregions 
except the root.

It's a tree of regions, each level adding an offset, clipping, and 
perhaps other attributes, with the leaves providing actual memory (mmio 
or RAM).

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