mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-12 05:24:12 +08:00
f4693c2716
For historical reasons, the arm64 kernel VA space is configured as two
equally sized halves, i.e., on a 48-bit VA build, the VA space is split
into a 47-bit vmalloc region and a 47-bit linear region.
When support for 52-bit virtual addressing was added, this equal split
was kept, resulting in a substantial waste of virtual address space in
the linear region:
48-bit VA 52-bit VA
0xffff_ffff_ffff_ffff +-------------+ +-------------+
| vmalloc | | vmalloc |
0xffff_8000_0000_0000 +-------------+ _PAGE_END(48) +-------------+
| linear | : :
0xffff_0000_0000_0000 +-------------+ : :
: : : :
: : : :
: : : :
: : : currently :
: unusable : : :
: : : unused :
: by : : :
: : : :
: hardware : : :
: : : :
0xfff8_0000_0000_0000 : : _PAGE_END(52) +-------------+
: : | |
: : | |
: : | |
: : | |
: : | |
: unusable : | |
: : | linear |
: by : | |
: : | region |
: hardware : | |
: : | |
: : | |
: : | |
: : | |
: : | |
: : | |
0xfff0_0000_0000_0000 +-------------+ PAGE_OFFSET +-------------+
As illustrated above, the 52-bit VA kernel uses 47 bits for the vmalloc
space (as before), to ensure that a single 64k granule kernel image can
support any 64k granule capable system, regardless of whether it supports
the 52-bit virtual addressing extension. However, due to the fact that
the VA space is still split in equal halves, the linear region is only
2^51 bytes in size, wasting almost half of the 52-bit VA space.
Let's fix this, by abandoning the equal split, and simply assigning all
VA space outside of the vmalloc region to the linear region.
The KASAN shadow region is reconfigured so that it ends at the start of
the vmalloc region, and grows downwards. That way, the arrangement of
the vmalloc space (which contains kernel mappings, modules, BPF region,
the vmemmap array etc) is identical between non-KASAN and KASAN builds,
which aids debugging.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Steve Capper <steve.capper@arm.com>
Link: https://lore.kernel.org/r/20201008153602.9467-3-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
172 lines
7.7 KiB
ReStructuredText
172 lines
7.7 KiB
ReStructuredText
==============================
|
|
Memory Layout on AArch64 Linux
|
|
==============================
|
|
|
|
Author: Catalin Marinas <catalin.marinas@arm.com>
|
|
|
|
This document describes the virtual memory layout used by the AArch64
|
|
Linux kernel. The architecture allows up to 4 levels of translation
|
|
tables with a 4KB page size and up to 3 levels with a 64KB page size.
|
|
|
|
AArch64 Linux uses either 3 levels or 4 levels of translation tables
|
|
with the 4KB page configuration, allowing 39-bit (512GB) or 48-bit
|
|
(256TB) virtual addresses, respectively, for both user and kernel. With
|
|
64KB pages, only 2 levels of translation tables, allowing 42-bit (4TB)
|
|
virtual address, are used but the memory layout is the same.
|
|
|
|
ARMv8.2 adds optional support for Large Virtual Address space. This is
|
|
only available when running with a 64KB page size and expands the
|
|
number of descriptors in the first level of translation.
|
|
|
|
User addresses have bits 63:48 set to 0 while the kernel addresses have
|
|
the same bits set to 1. TTBRx selection is given by bit 63 of the
|
|
virtual address. The swapper_pg_dir contains only kernel (global)
|
|
mappings while the user pgd contains only user (non-global) mappings.
|
|
The swapper_pg_dir address is written to TTBR1 and never written to
|
|
TTBR0.
|
|
|
|
|
|
AArch64 Linux memory layout with 4KB pages + 4 levels (48-bit)::
|
|
|
|
Start End Size Use
|
|
-----------------------------------------------------------------------
|
|
0000000000000000 0000ffffffffffff 256TB user
|
|
ffff000000000000 ffff7fffffffffff 128TB kernel logical memory map
|
|
[ ffff600000000000 ffff7fffffffffff ] 32TB [ kasan shadow region ]
|
|
ffff800000000000 ffff800007ffffff 128MB bpf jit region
|
|
ffff800008000000 ffff80000fffffff 128MB modules
|
|
ffff800010000000 fffffdffbffeffff 125TB vmalloc
|
|
fffffdffbfff0000 fffffdfffe5f8fff ~998MB [guard region]
|
|
fffffdfffe5f9000 fffffdfffe9fffff 4124KB fixed mappings
|
|
fffffdfffea00000 fffffdfffebfffff 2MB [guard region]
|
|
fffffdfffec00000 fffffdffffbfffff 16MB PCI I/O space
|
|
fffffdffffc00000 fffffdffffdfffff 2MB [guard region]
|
|
fffffdffffe00000 ffffffffffdfffff 2TB vmemmap
|
|
ffffffffffe00000 ffffffffffffffff 2MB [guard region]
|
|
|
|
|
|
AArch64 Linux memory layout with 64KB pages + 3 levels (52-bit with HW support)::
|
|
|
|
Start End Size Use
|
|
-----------------------------------------------------------------------
|
|
0000000000000000 000fffffffffffff 4PB user
|
|
fff0000000000000 ffff7fffffffffff ~4PB kernel logical memory map
|
|
[ fffd800000000000 ffff7fffffffffff ] 512TB [ kasan shadow region ]
|
|
ffff800000000000 ffff800007ffffff 128MB bpf jit region
|
|
ffff800008000000 ffff80000fffffff 128MB modules
|
|
ffff800010000000 fffff81ffffeffff 120TB vmalloc
|
|
fffff81fffff0000 fffffc1ffe58ffff ~3TB [guard region]
|
|
fffffc1ffe590000 fffffc1ffe9fffff 4544KB fixed mappings
|
|
fffffc1ffea00000 fffffc1ffebfffff 2MB [guard region]
|
|
fffffc1ffec00000 fffffc1fffbfffff 16MB PCI I/O space
|
|
fffffc1fffc00000 fffffc1fffdfffff 2MB [guard region]
|
|
fffffc1fffe00000 ffffffffffdfffff 3968GB vmemmap
|
|
ffffffffffe00000 ffffffffffffffff 2MB [guard region]
|
|
|
|
|
|
Translation table lookup with 4KB pages::
|
|
|
|
+--------+--------+--------+--------+--------+--------+--------+--------+
|
|
|63 56|55 48|47 40|39 32|31 24|23 16|15 8|7 0|
|
|
+--------+--------+--------+--------+--------+--------+--------+--------+
|
|
| | | | | |
|
|
| | | | | v
|
|
| | | | | [11:0] in-page offset
|
|
| | | | +-> [20:12] L3 index
|
|
| | | +-----------> [29:21] L2 index
|
|
| | +---------------------> [38:30] L1 index
|
|
| +-------------------------------> [47:39] L0 index
|
|
+-------------------------------------------------> [63] TTBR0/1
|
|
|
|
|
|
Translation table lookup with 64KB pages::
|
|
|
|
+--------+--------+--------+--------+--------+--------+--------+--------+
|
|
|63 56|55 48|47 40|39 32|31 24|23 16|15 8|7 0|
|
|
+--------+--------+--------+--------+--------+--------+--------+--------+
|
|
| | | | |
|
|
| | | | v
|
|
| | | | [15:0] in-page offset
|
|
| | | +----------> [28:16] L3 index
|
|
| | +--------------------------> [41:29] L2 index
|
|
| +-------------------------------> [47:42] L1 index (48-bit)
|
|
| [51:42] L1 index (52-bit)
|
|
+-------------------------------------------------> [63] TTBR0/1
|
|
|
|
|
|
When using KVM without the Virtualization Host Extensions, the
|
|
hypervisor maps kernel pages in EL2 at a fixed (and potentially
|
|
random) offset from the linear mapping. See the kern_hyp_va macro and
|
|
kvm_update_va_mask function for more details. MMIO devices such as
|
|
GICv2 gets mapped next to the HYP idmap page, as do vectors when
|
|
ARM64_HARDEN_EL2_VECTORS is selected for particular CPUs.
|
|
|
|
When using KVM with the Virtualization Host Extensions, no additional
|
|
mappings are created, since the host kernel runs directly in EL2.
|
|
|
|
52-bit VA support in the kernel
|
|
-------------------------------
|
|
If the ARMv8.2-LVA optional feature is present, and we are running
|
|
with a 64KB page size; then it is possible to use 52-bits of address
|
|
space for both userspace and kernel addresses. However, any kernel
|
|
binary that supports 52-bit must also be able to fall back to 48-bit
|
|
at early boot time if the hardware feature is not present.
|
|
|
|
This fallback mechanism necessitates the kernel .text to be in the
|
|
higher addresses such that they are invariant to 48/52-bit VAs. Due
|
|
to the kasan shadow being a fraction of the entire kernel VA space,
|
|
the end of the kasan shadow must also be in the higher half of the
|
|
kernel VA space for both 48/52-bit. (Switching from 48-bit to 52-bit,
|
|
the end of the kasan shadow is invariant and dependent on ~0UL,
|
|
whilst the start address will "grow" towards the lower addresses).
|
|
|
|
In order to optimise phys_to_virt and virt_to_phys, the PAGE_OFFSET
|
|
is kept constant at 0xFFF0000000000000 (corresponding to 52-bit),
|
|
this obviates the need for an extra variable read. The physvirt
|
|
offset and vmemmap offsets are computed at early boot to enable
|
|
this logic.
|
|
|
|
As a single binary will need to support both 48-bit and 52-bit VA
|
|
spaces, the VMEMMAP must be sized large enough for 52-bit VAs and
|
|
also must be sized large enough to accommodate a fixed PAGE_OFFSET.
|
|
|
|
Most code in the kernel should not need to consider the VA_BITS, for
|
|
code that does need to know the VA size the variables are
|
|
defined as follows:
|
|
|
|
VA_BITS constant the *maximum* VA space size
|
|
|
|
VA_BITS_MIN constant the *minimum* VA space size
|
|
|
|
vabits_actual variable the *actual* VA space size
|
|
|
|
|
|
Maximum and minimum sizes can be useful to ensure that buffers are
|
|
sized large enough or that addresses are positioned close enough for
|
|
the "worst" case.
|
|
|
|
52-bit userspace VAs
|
|
--------------------
|
|
To maintain compatibility with software that relies on the ARMv8.0
|
|
VA space maximum size of 48-bits, the kernel will, by default,
|
|
return virtual addresses to userspace from a 48-bit range.
|
|
|
|
Software can "opt-in" to receiving VAs from a 52-bit space by
|
|
specifying an mmap hint parameter that is larger than 48-bit.
|
|
|
|
For example:
|
|
|
|
.. code-block:: c
|
|
|
|
maybe_high_address = mmap(~0UL, size, prot, flags,...);
|
|
|
|
It is also possible to build a debug kernel that returns addresses
|
|
from a 52-bit space by enabling the following kernel config options:
|
|
|
|
.. code-block:: sh
|
|
|
|
CONFIG_EXPERT=y && CONFIG_ARM64_FORCE_52BIT=y
|
|
|
|
Note that this option is only intended for debugging applications
|
|
and should not be used in production.
|