In Intel's 5-Level Paging and 5-Level EPT white paper, they planed to introduce another level paging mechanism, PML5, extending the linear address from 48-bit to 57-bit (57-bit, not 64-bit!!!!!). What that stupid thing that I find on earth! Adding a level to current paging system is easy and simple, but the overheads it brings do really waste the treasurable memory resources in doing nothing at all, but increase the power consumption too. Are they really make it innovative or another experimental product letting the end consumers to pay for the bill? Remember Intel Pentium 4 processors? For their poor performance, but incredible power hunger and heat producing? Stupid NetBurst microarchitecture consumed too lot of energies but almost doing nothing at all, but people then paid lot of money onto them, and they might just only paid for the costs while they do research on the Intel Core microarchitecture.
What about AMD? I love AMD64 processors, because the AMD64 architecture is a practical and feasible architecture meeting today's needs. This architecture also provided the back-ward support for the legacy x86 software in the compatibility and Legacy mode. If future AMD64 processors with Legacy Mode eliminated, then it would get rid of lots of limitations. I wish AMD64 architecture is a smart and smarter architecture, I wish the AMD architects would use some other smarter ways to implement future paging mechanism, rather than merely introducing additional paging level.
256 Tbyte Virtual Adressspace should be enough just like 640 Kbyte of Ram is enough for everyone.
If i want to map my single 16 Tbyte HDD into my VA i already use 2^44 Bits.
I'm not talking about any special hardware only a hand full of the biggest companies world wide have. I'm talking about my single Disk usecase. And with ~600€ for a single DIsk this usecase is affordable for Homeusers.
Imagine someone wants a Raid 6 with 4 of those disks... already 2^46 Bits VA used.
Imagine an application which wants to map 16 Harddisks with 16 Tbyte each into a single VA...
Then you will find out, you already passed the limits.
I agree, that in most usecases today it is not necessary and it should stay disabled by default.
An additional Level of paging can cost some power/performance and if properly designed maybe 4 to 8 KByte of Ram....
But if i could use 57 Bits VA by today i would have a lot less headache.