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

[Peter Maydell]

On 14 January 2014 09:20, Alexander Graf <address@hidden> wrote:
> Of course there's potential for future extensions to allow for more
> bits in the future, but at least the current generation x86_64 (and x86)
> specification clearly only supports 52 bits of physical address space.
> And non-x86(_64) don't care about bigger address spaces either
> because they use BAR windows which are very unlikely to grow
> bigger than 52 bits ;).

There's no reason you couldn't do an ARM (most likely AArch64)
system which dealt with PCI BARs the same way as x86 rather
than having a fixed window in the memory map; I wouldn't be
surprised if some of the server designs took that route. However
the architecture specifies a 48 bit maximum physical address.

With some of the BAR-window design PCI controllers I think it's
theoretically possible to configure the controller so that the
window shows the very top part of PCI address space and then
configure all your device BARs with very high PCI addresses.
In that case the BAR MemoryRegions would get mapped in
at very high addresses in the PCI memory address space
MemoryRegion container, and at more usual small addresses
in the system AddressSpace.

thanks
-- PMM