On Sat, 2007-09-29 at 23:43 +0100, Paul Brook wrote: > > > Also note that changing variables from int to long have strictly no > > > impact on 32 bits host machines, then won't help emulating more than 2 > > > GB of RAM. Another variable type (target_phys_addr_t ?) should be used > > > instead. > > > > This patch should be restricted to 64-bit hosts. I don't think it's > > useful to emulate a 64-bit target with huge amounts of virtual and > > physical address space on a 32-bit host.
My feeling is that if it's restricted to 64 bits host, then it's a patch for geeks only, that brings no useful feature to the main end-users. In the real world, most people are still running in 32 bits mode. > IMHO Huge amounts of virtual address space can definitely be useful, even if > you don't have ram to back it. > > Huge amounts of physical address space is less immediately useful, though in > practice you have to emulate whatever real hardware provides. If you're > emulating a machine with a 40+ bit physical address space, there's a fair > chance your guest OS will decide to scatter a relatively small set of > resources over the whole address space. I don't agree too much with your opinion, because what I can see is that PowerPC 64 machines (at least IBM ones) tend to use the 62 bits physical address space provided by the architecture. If I remember well, there is at least one PPC64 architecture where the highest bits are used to split the physical address space between memory, memory-mapped IO, devices, ... I'm quite sure there are other 64 bits architecture that have the same requirement of a huge physical address space, then beeing able to handle it in Qemu seems to be very useful, much more than trying to emulate a huge amount of RAM, and is needed in a very near future. > I agree there's no point trying to emulate >2G ram on a 32-bit host, but > physical address space and ram are two very different things. > For example I have a cpu that has a "bitbanded" memory region. This expands > each bit of real ram to a whole 32-bit word, effectively turning a word > load/store into an atomic bit operation. Currently it's only used for > relatively small address ranges, but it's a good example of a situation where > the physical address space is much larger than ram. I don't see why it would be useless to emulate huge amount of RAM on 32 bits hosts. If you try to register more than a few gigabytes of memory, there are great chances that the host machine won't have the physical RAM to handle it at once, so a page swap mechanism will have to be implemented. Then, I see no difference in using it on a 32 bits hosts or a 64 bits ones. Regards. -- J. Mayer <[EMAIL PROTECTED]> Never organized
