Intel 5-level paging: Difference between revisions

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Revision as of 23:34, 11 March 2020

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{{Use dmy dates|date=August 2018}}

[[File:Page Tables (5 levels).svg|thumb|A diagram of five levels of paging]]

'''Intel 5-level paging''', referred to simply as ''5-level paging'' in [[Intel]] documents, is a processor extension for the [[x86-64]] line of processors.<ref name="intel-white-paper">{{Cite web|url=https://software.intel.com/sites/default/files/managed/2b/80/5-level_paging_white_paper.pdf|title=5-Level Paging and 5-Level EPT|publisher=Intel Corporation|date=May 2017}}</ref>{{Rp|11}} It extends the size of [[virtual address]]es from 48&nbsp;bits to 57&nbsp;bits, increasing the addressable [[virtual memory]] from 256&nbsp;[[terabyte|terabytesTiB]] to 128&nbsp;[[petabyte|petabytesPiB]].<ref group=NB>{{BDprefix|p=B}}</ref> While the technical document describing the extension is a [[white paper]], stating ''"do not finalize a design with this information",'' support for the extension has already been implemented in the [[Linux kernel]].<ref>{{Cite news|url=https://www.zdnet.com/article/first-linux-4-14-release-adds-very-core-features-arrives-in-time-for-kernels-26th-birthday/|title=First Linux 4.14 release adds "very core" features, arrives in time for kernel's 26th birthday {{!}} ZDNet|last=Tung|first=Liam|work=ZDNet|access-date=2018-04-25|language=en}}</ref>

== Technology ==

Current [[x86-64]] processors use a four-level page table structure when operating in 64-bit mode.<ref name="x86-software-developers-manual" />{{Rp|2806}} A similar situation arose when the 32 bit [[IA-32]] processors used two levels, allowing up to four [[Gigabyte|gigabytesGiB]] of memory (both virtual and physical). To support more than 4&nbsp;GB of [[RAM]], an additional mode of address translation called [[Physical Address Extension]] (PAE) was defined, involving a third level.<ref>{{Cite web|url=https://docs.microsoft.com/en-us/previous-versions/windows/hardware/design/dn613969(v=vs.85)|title=Operating Systems and PAE Support - Windows 10 hardware dev|last=Hudek|first=Ted|website=docs.microsoft.com|language=en-us|access-date=2018-04-26}}</ref> This was enabled by setting a bit in [[Control register#CR4|the CR4 register]].<ref name="x86-software-developers-manual" />{{Rp|2799}} Likewise, the new extension is enabled by setting bit 12 of the CR4 register (known as LA57).<ref name="intel-white-paper" />{{Rp|16}} This is only used when the processor is operating in 64 bit mode, and only may be modified when it is not.<ref name="intel-white-paper" />{{Rp|16}} If the bit is not set, the processor operates with four paging levels.

The new extensions allow up to 4&nbsp;PB of physical memory,<ref name="phoronix" /> compared to the maximum of 256&nbsp;TB on previous processors.<ref name="amd10h">{{cite web|url=http://developer.amd.com/wordpress/media/2012/10/31116.pdf|title=BIOS and Kernel Developer's Guide (BKDG) For AMD Family 10h Processors|page=24|accessdate=2018-04-26|quote=Physical address space increased to 48 bits.}}</ref> As adding another page table multiplies the address space by 512, the virtual limit has increased from 256&nbsp;TB to 128&nbsp;PB. An extra nine bits of the virtual address index the new table, so while formerly bits&nbsp;0 through&nbsp;47 were in use, now bits&nbsp;0 through&nbsp;56 are in use.

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== Drawbacks ==

Adding another level of indirection makes [[page table]] "walks" longer.<ref>{{Cite book|title=MICRO-50 : the 50th Annual IEEE/ACM International Symposium on Microarchitecture : proceedings : October 14-18, 2017, Cambridge, MA|others=Institute of Electrical and Electronics Engineers., IEEE Computer Society., ACM Special Interest Group on Microprogramming|isbn=9781450349529|___location=New York, New York|oclc=1032337814|date = 14 October 2017}}</ref> A page table walk occurs when either the processor's [[memory management unit]] or the memory management code in the operating system navigates the tree of page tables to find the [[page table entry]] corresponding to a virtual address.<ref>{{Cite web|url=http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0301h/I1026235.html|title=ARM Information Center|website=infocenter.arm.com|access-date=2018-04-26}}</ref><ref name="x86-software-developers-manual">{{Cite book|url=https://software.intel.com/sites/default/files/managed/39/c5/325462-sdm-vol-1-2abcd-3abcd.pdf|title=Intel® 64 and IA-32 Architectures Software Developer's Manual|last=|first=|publisher=[[Intel Corporation]]|year=2018|isbn=|___location=|pages=}}</ref>{{Rp|2806}} This means that, in the worst case, the processor or the memory manager has to access physical memory six times for a single virtual memory access, rather than five for the previous iteration of x86-64 processors. This results in slightly reduced memory access speed.<ref name="cse-451-paging-tlbs-slides" /> In practice this cost is greatly mitigated by caches such as the [[translation lookaside buffer]] (TLB).<ref name="cse-451-paging-tlbs-slides">{{Cite web|url=https://courses.cs.washington.edu/courses/cse451/08au/lectures/10-paging_TLBs.pdf|title=CSE 451: Operating Systems: Paging & TLBs|last=Levy|first=Hank|authorlink=Hank Levy (computer scientist)|date=Autumn 2008|website=[[University of Washington]]|access-date={{date|2018-04-26}}}}</ref>

== Notes ==

{{reflist|group=NB}}

== References ==