The document describes the ARMv7-A architecture and its support for large physical addresses (LPAE) in Linux. Key points include: - ARMv7-A supports LPAE through a 3-level translation table that maps 40-bit virtual addresses to 40-bit physical addresses. - Linux implements LPAE by modifying page table definitions, extending the swapper page directory to cover three levels, and adapting functions for setting page table entries and switching address spaces. - Low memory is mapped with 2MB sections while page tables can be allocated from high memory. Exception handling and PGD allocation/freeing were also updated for LPAE.

1. 1
ARMv7-A Architecture
Overview
David Brash
Architecture Program Manager, ARM Ltd.

2. 2
ARM Architecture roadmap
Announced Aug-2010:
• Large PhysAddr Extn
• PA => 40-bits
•Virtualization Extn
(v8 builds on these)

3. 3
ARMv7-A registers
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13 (SP)
R14 (LR)
R0 R0 R0 R0 R0R0
SP_svc
LR_svc
SP_irq
LR_irq
SP_und
LR_und
SP_fiq
LR_fiq
SP_abt
LR_abt
R8_fiq
R9_fiq
R10_fiq
R11_fiq
R12_fiq
SP_hyp
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R1
R2
R3
R4
R5
R6
R7
LR_(BL)
CPSR bits
• Flags: NZCVQ
• IT[7:0] status bits
• GE[3:0] Adv SIMD flags
• J (Jazelle)
• T (Thumb)
• E (endian)
• I, A, F masks
• M[4:0] (mode)
SPSR_svc SPSR_abt SPSR_und SPSR_irq SPSR_hyp
ELR_hyp
SPSR_fiq
CPSR

4. 4
Classic ARM MMU
 32-bit physical address space
 2-level translation tables
 Pointed to by TTBR0 (user mappings) and TTBR1 (kernel mappings
but with restrictions to the user/kernel memory split)
 32-bit page table entries
 1st
level contains 4096 entries (4 pages for PGD)
 1MB section per entry or
 Pointer to a 2nd
level table
 Implementation-defined 16MB supersections
 2nd
level contains 256 entries pointing to 4KB page each
 1KB per 2nd
level page table
 ARMv6/v7 introduced TEX remapping
 Memory type becomes a 3-bit index

5. 5
Classic ARM MMU (cont’d)
 Other features
 XN (eXecute Never) bit
 Different memory types: Normal (cacheable and non-cacheable),
Device, Strongly Ordered
 Shareability attributes for SMP systems
 ASID-tagged TLB (ARMv6 onwards)
 Avoids TLB flushing at context switch
 8-bit ASID value assigned to an mm_struct
 Dynamically allocated (there can be more than 256 processes)

6. 6
Classic ARM MMU Limitations
 Only 32-bit physical address space
 Growing market requiring more than 4GB physical address space
(both RAM and peripherals)
 Supersections can be used to allow up to 40-bit addresses using
16MB sections (implementation-defined feature)
 Prior to ARMv6, not a direct link between access permissions
and Linux PTE bits
 Simplified permission model introduced with ARMv6 but not used by
Linux
 2nd
level page table does not fill a full 4K page
 ARM Linux workarounds
 Separate array for the Linux PTE bits
 1st
level entry consists of two 32-bit locations pointing to 2KB 2nd
level
page table entries

7. 7
The ARMv7-A Virtualization Extensions
Popek and Goldberg summarized the concept in 1974:
 Equivalence/Fidelity
 A program running under the hypervisor should exhibit a behaviour
essentially identical to that demonstrated when running on an
equivalent machine directly.
 Resource control / Safety
 The hypervisor should be in complete control of the virtualized
resources.
 Efficiency/Performance
 A statistically dominant fraction of machine instructions must be
executed without hypervisor intervention.
"Formal Requirements for Virtualizable Third Generation Architectures". Communications of the ACM 17

8. 8
ARM hypervisor support philosophy
 Virtual machine (VM) scheduling and resource sharing
 New Hyp mode for Hypervisor execution
 Minimise Hypervisor intervention for “routine” GuestOS tasks
 Guest OS page table management
 Interrupt control
 Guest OS Device Drivers
 Syndrome support for trapping key instructions
 GuestOS load/store emulation
 Privileged control instructions
 System instructions (MRS, MSR) to read/write key registers
 Virtualized ID register management

9. 9
ARM virtualization extension - modes
A new privilege layer – hypervisor mode
 Guest OS kernel given same privilege structure as for a non-
virtualized environment
 Can run the same instructions
 Hypervisor mode has higher privilege
 VMM can control a wide range of OS accesses to hardware
User Mode
(Non-privileged)
Supervisor Mode
(Privileged)
Hyp Mode
(More Privileged)
Guest Operating System1
App2App1
Guest Operating System2
App2App1
Virtual Machine Monitor (VMM) or
Hypervisor

10. 10
Hypervisor mode and security extensions
Hypervisor mode applies to normal world
Secure world supports a single virtual machine
Monitor mode controls transition between worlds
Guest Operating System1
App2App1
Guest Operating System2
App2App1
Virtual Machine Monitor (VMM) or
Hypervisor
TrustZone Monitor
Secure World OS
Trusted App2Trusted App1

11. 11
ARMv7-A Virtualization - key features 1
 Trap and control support (HYP mode)
 Rich set of trap options (TLB/cache ops, ID groups, instructions)
 Syndrome register support
 EC: exception class (instr type, I/D abort into/within HYP)
 IL: instruction length (0 == 16-bit; 1 == 32-bit)
 ISS: instruction specific syndrome (instr fields, reason code, ...)
3
1
2
6
2
5
2
4
0
EC I
L
ISS

12. 12
ARMv7-A Virtualization - key features 2
 Dedicated Exception Link Register (ELR)
 Stores preferred return address on exception entry
 New instruction – ERET – for exception return from HYP mode
 Other modes overload exception model and procedure call LRs
 R14 used by exception entry BL and BLX instructions
 Address translation

13. 13
Two layers of address translation
Stage 1 translation
owned by guest OS
Virtual address map Intermediate physical
address map
Real System Physical
address map
Stage 2 translation
owned by the VMM
Hardware has 2-stage
memory translation
Tables from Guest OS
translate VA to IPA
Second set of tables from
VMM translate IPA to PA
Allows aborts to be routed to
appropriate software layer

14. 14
LPAE – Stage 1 (VA => {I}PA)
 Managed by the OS
 64-bit descriptors, 512 entries per table
 4KB table size == 4KB page size
 1 or 2 Translation Table Base Registers
 3 levels of table supported
 up to 9 address bits per level
 2 bits at Level 1
 9 bits at Levels 2 and 3
 Page Table Entry:
32-bitVirtual
Address
TTBR1
space
TTBR0
space
Not mapped
(Fault)
232- TTCR.T1SZ
232-TTCR.T0SZ
0
232
T1SZ ≤ T0SZ
If T1SZ == T0SZ == 0 then TTBR0 always used
31:30 VA Bits <29:21> VA Bits <20:12> VA Bits <11:0>
L1 Level 2 table index Level 3 table (page) index Page offset address
6
3
5
2
4
0
3
0
1
2
2 1 0
Upper attributes SBZ Address out SBZ Lower attributes
Valid
Block/TableBlock/page
• Memory type
• Cache policy
• Shareability, AF, nG, NS flags
• Access permissions
•Table override bits (NS, XN, PXN, AP[1:0])
• Block/Page XN, PXN bits
• Block/Page 16x entry contiguous hint
• IGNORED bits

15. 15
LPAE – Stage 2 (IPA => PA)
 Managed by the VMM / hypervisor
 Same table walk scheme as stage 1, now up to 40-bit input address:
 2x contiguous 4KB tables allowed at L1; support 240
address space
 Up to 1-16 contiguous tables allowed at L2; support 230
- 234
address space
 1x Translation Table Base register (VTBR)
 Page table entry:
6
3
5
2
4
0
3
0
1
2
2 1 0
Upper attributes SBZ Address out SBZ Lower attributes
Valid
Block/Table
Block / page only
• XN bit
• 16x entry contiguous hint
• IGNORED bits
Block/page
• Memory type
• Cache policy
• Shareability, AF bit
• R/W Access permissions
3
9
3
4
3
0
2
1
1
2
0
IPA Bits <39:30> IPA Bits <29:21> IPA Bits <20:12> IPA Bits <11:0>
IPA Bits <11:0> IPA Bits <33;21> IPA Bits <20:12> IPA Bits <11:0>

16. 16
Linux Support for
ARM LPAE
Catalin Marinas
ELC Europe 2011

17. 17
ARM LPAE Features
 40-bit physical addresses (1TB)
 40-bit intermediate physical addresses (guest PA space)
 3-level translation tables
 Pointed to by TTBR0 (user mappings) and TTBR1 (kernel mappings)
 Not as restrictive on user/kernel memory split (can use 3:1)
 With 1GB kernel mapping, the 1st
level is skipped
 64-bit entries in each level
 1st
level contains 4 entries (stage 1 translation)
 1GB section or
 Pointer to 2nd
level table
 2nd
level contains 512 entries (4KB in total)
 2MB section or
 Pointer to 3rd
level

18. 18
ARM LPAE Features (cont’d)
 3rd
level contains 512 entries (4KB)
 Each addressing a 4KB range
 Possibility to set a contiguity flag for 16 consecutive pages
 LDRD/STRD (64-bit load/store) instructions are atomic on
ARM processors supporting LPAE
 Only the simplified page permission model is supported
 No kernel RW and user RO combination
 Domains are no longer present (they have already been
removed in ARMv7 Linux)
 Additional spare bits to be used by the OS
 Dedicated bits for user, read-only and access flag (young)
settings

19. 19
ARM LPAE Features (cont’d)
 ASID is part of the TTBR0 register
 Simpler context switching code (no need to deal with speculative TLB
fetching with the wrong ASID)
 The Context ID register can be used solely for debug/trace
 Additional permission control
 PXN – Privileged eXecute Never
 SCTLR.WXN, SCTLR.UWXN – prevent execution from writable
locations (the latter only for user accesses)
 APTable – restrict permissions in subsequent page table levels
 XNTable, PXNTable – override XN and PXN bits in subsequent page
table levels
 New registers for the memory region attributes
 MAIR0, MAIR1 – 32-bit Memory Attribute Indirection Registers
 8 memory types can be configured at a time

20. 20
ARM LPAE Features
1GB block
Table ptr
1st level
table
VA[31:30]
TTBRx
4KB
2MB block
Table ptr
2nd level
table
VA[29:21]
4KB
PTE
PTE
3rd level
table
VA[20:12]
4KB
64-bit entry 64-bit entry64-bit entry
PGD PMD PTE

21. 21
Linux and ARM LPAE
 Linux + ARM LPAE has the same memory layout as the
classic MMU implementation
 Described in Documentation/arm/memory.txt
 0..TASK_SIZE – user space
 PAGE_OFFSET-16M..PAGE_OFFSET-2M – module space
 PAGE_OFFSET-2M..PAGE_OFFSET – highmem mappings
 Highmem is already supported by the classic MMU
 Memory beyond 4G is only accessible via highmem
 Page mapping functions use pfn (32-bit variable, PAGE_SHIFT == 12,
maximum 44-bit physical addresses)
 Original ARM kernel port assumed 32-bit physical addresses
 LPAE redefines phys_addr_t, dma_addr_t as u64 (generic typedefs)
 New ATAG_MEM64 defined for specifying the 64-bit memory layout

22. 22
Linux and ARM LPAE (cont’d)
 Hard-coded assumptions about 2 levels of page tables
 PGDIR_(SHIFT|SIZE|MASK) references converted to PMD_*
 swapper_pg_dir extended to cover both 1st
and 2nd
levels of
page tables
 1 page for PGD (only 4 entries used for stage 1 translations)
 4 pages for PMD
 init_mm.pgd points to swapper_pg_dir
 TTBR0 used for user mappings and always points to PGD
 TTBR1 used for kernel mapping:
 3:1 split – TTBR1 points to 4th
page of PMD (2 levels only, 1GB)
 Classic MMU does not allow the use of TTBR1 for the 3:1 split
 2:2 split – TTBR1 points to 3rd
PGD entry
 1:3 split – TTBR1 points to 1st
PGD entry

23. 23
Linux and ARM LPAE (cont’d)
 The page table definitions have been separated into
pgtable*-2level.h and pgtable*-3level.h files
 Few PTE bits shared between classic and LPAE definitions
 Negated definitions of L_PTE_EXEC and L_PTE_WRITE to match the
corresponding hardware bits
 Memory types are the same and they represent an index in the TEX
remapping registers (PRRR/NMRR or MAIR0/MAIR1)
 The proc-v7.S file has been duplicated into proc-v7lpae.S
 Different register setup for TTBRx and TEX remapping (MAIRx)
 Simpler cpu_v7_set_pte_ext (1:1 mapping between hardware and
software PTE bits)
 Simpler cpu_v7_switch_mm (ASID switched with TTBR0)
 Current ARM code converted to pgtable-nopud.h
 Not using „nopmd‟ with classic MMU

24. 24
Linux and ARM LPAE (cont’d)
 Lowmem is mapped using 2MB sections in the 2nd
level table
 PGD and PMD only allocated from lowmem
 PTE tables can be allocated from highmem
 Exception handling
 Different IFSR/DFSR registers structure and exception numbering –
arch/arm/mm/fault.c modified accordingly
 Error reporting includes PMD information as well
 PGD allocation/freeing
 Kernel PGD entries copied to the new PGD during pgd_alloc()
 Modules and pkmap entries added to the PMD during fault handling
 Identity mapping (PA == VA)
 Required for enabling or disabling the MMU – secondary CPU
booting, CPU hotplug, power management, kexec

25. 25
Linux and ARM LPAE (cont’d)
 Uses pgd_alloc() and pgd_free()
 When PHYS_OFFSET > PAGE_OFFSET, kernel PGD entries may be
overridden
 swapper_pg_dir entries marked with an additional bit –
L_PGD_SWAPPER
 pgd_free() skips such entries during clean-up

26. 26
Current Status and Future
 Initial development done on software models
 Tested on real hardware (FPGA and silicon)
 Parts of the LPAE patch set already in mainline
 Mainly preparatory patches, not core functionality
 Aiming for full support in Linux 3.3
 Hardware supporting LPAE
 ARM Cortex-A15, Cortex-A7 processors
 TI OMAP5 (dual-core ARM Cortex-A15)
 Other developments
 KVM support for Cortex-A15 – implemented by Christoffer Dall at
Columbia University
 Uses the Virtualisation extensions together with the LPAE stage 2
translations

27. 27
Reference
 ARM Architecture Reference Manual rev C
 Currently beta, not publicly available yet
 Specifications publicly available on ARM Infocenter
 http://infocenter.arm.com/
 ARM architecture -> Reference Manuals -> ARMv7-AR LPA
Virtualisation Extensions
 Linux patches – ARM architecture development tree
 Hosts the latest ARM architecture developments before patches are
merged into the mainline kernel
 git://github.com/cmarinas/linux.git
 When the kernel.org accounts are back
 git://git.kernel.org/pub/scm/linux/cmarinas/linux-arm-arch.git

28. 28
ARM Architecture
System Features

29. 29
Interrupt Controller
 ARM standardising on the “Generic Interrupt Controller” [GIC]
 Supports the ARMv7 Security Extension
 Interrupt groups support (Nonsecure) IRQ and (Secure) FIQ split
 Distributor and cpu interface functionality
 GICv2 adds support for a virtualized cpu interface
 VMM manages physical interface & queues entries for each VM.
 GuestOS (VM) configures, acknowledges, processes and completes
interrupts directly. Traps to VMM where necessary.
 Hypervisor can virtualize FIQs from the physical (Nonsecure, IRQ)
interface.

30. 30
Interrupt Handling – GICv2
Secure handler
(monitor mode)
or
OS FIQ handler
OS IRQ handler
GuestOS IRQ handler
GuestOS FIQ handler
VirtualCpu InterfaceN
• cpu-specific control + state
• Interrupt ACK / END (retire)
Virtual Grp0: FIQ
Virtual Grp1: IRQ
Hypervisor – virtualized distributor
• Grp1 physical => Grp1/Grp0 virtual
Virtual interrupt management
• interrupt queues (list registers)
Virtualization support
* ARM Architecture Security Extns
NEW
Distributor - control and configuration
• Assignment (≤8 cpu interfaces)
• Interrupt grouping (Grp0, Grp1)
Grp0: (Secure*, FIQ)
Grp1: IRQ always
Cpu Interface0
• cpu-specific control + state
• Interrupt ACK / END (retire)
Grp0: (Secure*, FIQ)
Grp1: IRQ always
GICv1
Cpu InterfaceN
• cpu-specific control + state
• Interrupt ACK / END (retire)
• software
• private (per cpu) peripheral
• shared peripheral
Interrupt sources:

31. 31
Generic Timer
 Shared “always on” counter
 Fast read access for reliable
distribution of time.
 ≥ ComparedValue or ≤ 0 timer
event configuration support
 Maskable interrupts
 Offset capability for virtual time
 Event stream support
 Hypervisor trap support
SoC
Counter
Power
Controller
CPU Cluster1
CPU0
Timer0
CPU1
Timer1
Counter Interface
Memory Interconnect and Memory Controller
Shared Cache
$ $
Timer Distribution
Bus
Peripheral Bus
Always Powered
Power Domain
GIC
CPU Cluster0
CPU0
Timer0
CPU1
Timer1
Counter Interface
Shared Cache
$ $
GIC
Systemevents
Implementation example:

32. 32
System MMU
Local Coherence Bus (no snooping on
bus)
“Event pulse” enabling next cycle
notifications
 System MMU architecture translation options from pre-set tables
 No Translation
 VA -> IPA (Stage 1) or IPA->PA (Stage 2) only
 VA->PA (Stage1 and Stage2)
 Can share page tables with ARM cores – relate contexts to VMIDs/ASIDs
L1
I-cache
L1
D-cache
Core0
L1
I-cache
L1
D-cache
CoreN
L2 cache
System interconnect
DMA
System MMU System Memory

33. 33
Quad
Cortex-A15
Quad
Cortex-A15
AMBA 4 Cache Coherent Interconnect
CCI-400
I/O
device
MMU-400
DMC-400
Dynamic Memory Controller
AXI Network
Interconnect
Slaves Slaves
AXI Network
Interconnect
LCDVideo
DDR3/
LPDDR2
DDR3/
LPDDR2
PHY
GIC-400
Mali
Graphics
PHY
MMU-400 MMU-400
Completing the SoC
IPA
PA
Intermediate Physical Address
(IPA)
The address the OS believes
it‟s accessing
Physical Address (PA)
The actual address the VMM
assigns

34. 34
To summarize: ARMv7-A additions
 A natural progression of well established features applied to
an architecture long associated with low power.
 „Classic‟ uses and solutions will apply to ARM markets:
 Low cost/low power server opportunities
 Application OS + RTOS/microkernel smart mobile devices
 Feature split by „Manufacturers‟ versus User‟ VM environment
 Automotive, home, ...
 Opportunities for new use cases?
 Today‟s entrepreneurs/ideas
=> tomorrow‟s major businesses

35. 35
Thank you