Re: [Qemu-devel] [PULL 14/28] exec: make address spaces 64-bit wide

[Mike Day]

On Tue, Jan 14, 2014 at 12:49 PM, Mike Day <address@hidden> wrote:
>>> > > >>>>>>>
>>> > > >>>>>>>>> Prior to this change, there was no re-map with the 
>>> > > >>>>>>>>> fffffffffebe0000
>
>> If we choose not to map them, how do we distinguish them from guest RAM?
>> There's no MemoryRegion flag that I'm aware of to distinguish a ram_ptr
>> that points to a chunk of guest memory from one that points to the mmap
>> of a device BAR.  I think I'd need to explicitly walk all of the vfio
>> device and try to match the MemoryRegion pointer to one of my devices.
>> That only solves the problem for vfio devices and not ivshmem devices or
>> pci-assign devices.
>>
>
> I don't know if this will save you doing your memory region search or
> not. But a BAR that ends with the low bit set is MMIO, and BAR that
> ends with the low bit clear is RAM. So the address above is RAM as was
> pointed out earlier in the thread. If you got an ambitious address in
> the future you could test the low bit. But MMIO is deprecated
> according to http://wiki.osdev.org/PCI so you probably won't see it,
> at least for 64-bit addresses.

s/ambitious/ambiguous/

The address above has already been masked. What you need to do is read
the BAR. If the value from the BAR end in '1', its MMIO. If it ends in
'10', its RAM. If it ends in '0n' its disabled. The first thing that
the PCI software does after reading the BAR is mask off the two low
bits.

Mike