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

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-05-18 17:37, Avi Kivity wrote:
>>> void memory_region_init_ram(MemoryRegion *mr,
>>> target_phys_addr_t size);
>>> void memory_region_init_ram_ptr(MemoryRegion *mr,
>>> target_phys_addr_t size,
>>> void *ptr);
>>> void memory_region_destroy(MemoryRegion *mr);
>>> void memory_region_set_offset(MemoryRegion *mr, target_phys_addr_t
>>> offset);
>>
>> What's "offset" mean?
> 
> It's the equivalent of cpu_register_physical_memory_offset().
> 
> Note the intent is to have addresses always be relative to the innermost
> container.  Perhaps we can get away without offset.

Offset is supposed to support the transition of devices that expect
absolute I/O addresses in their callbacks to those that are fine with
relative ones (based on the region start). This API change is a good
chance to finally get rid of the former group.

> 
>>
>>> void memory_region_set_log(MemoryRegion *mr, bool log);
>>> void memory_region_clear_coalescing(MemoryRegion *mr);
>>> void memory_region_add_coalescing(MemoryRegion *mr,
>>> target_phys_addr_t offset,
>>> target_phys_addr_t size);
>>
>> I don't think it's worth while to try to fit coalescing into this API.
>> It's a KVM specific hack.  I think it's fine to be a hacked on API.
> 
> The problem is that only the device knows about coalescing, while the
> region can be mapped, unmapped, or moved without device knowledge.  So
> if a PCI is unmapped and then remapped (possibly at a different address)
> we need to tell kvm about it, but there is no device callback involved.

Doesn't Xen use coalescing as well, or could use? It looks like a
generic optimization feature for hypervisors that want to build on top
of QEMU.

Jan


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