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linux/Documentation/arm64/elf_hwcaps.rst
Linus Torvalds e9a83bd232 It's been a relatively busy cycle for docs: 
- A fair pile of RST conversions, many from Mauro.  These create more
    than the usual number of simple but annoying merge conflicts with other
    trees, unfortunately.  He has a lot more of these waiting on the wings
    that, I think, will go to you directly later on.
 
  - A new document on how to use merges and rebases in kernel repos, and one
    on Spectre vulnerabilities.
 
  - Various improvements to the build system, including automatic markup of
    function() references because some people, for reasons I will never
    understand, were of the opinion that :c:func:``function()`` is
    unattractive and not fun to type.
 
  - We now recommend using sphinx 1.7, but still support back to 1.4.
 
  - Lots of smaller improvements, warning fixes, typo fixes, etc.
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Merge tag 'docs-5.3' of git://git.lwn.net/linux

Pull Documentation updates from Jonathan Corbet:
 "It's been a relatively busy cycle for docs:

   - A fair pile of RST conversions, many from Mauro. These create more
     than the usual number of simple but annoying merge conflicts with
     other trees, unfortunately. He has a lot more of these waiting on
     the wings that, I think, will go to you directly later on.

   - A new document on how to use merges and rebases in kernel repos,
     and one on Spectre vulnerabilities.

   - Various improvements to the build system, including automatic
     markup of function() references because some people, for reasons I
     will never understand, were of the opinion that
     :c:func:``function()`` is unattractive and not fun to type.

   - We now recommend using sphinx 1.7, but still support back to 1.4.

   - Lots of smaller improvements, warning fixes, typo fixes, etc"

* tag 'docs-5.3' of git://git.lwn.net/linux: (129 commits)
  docs: automarkup.py: ignore exceptions when seeking for xrefs
  docs: Move binderfs to admin-guide
  Disable Sphinx SmartyPants in HTML output
  doc: RCU callback locks need only _bh, not necessarily _irq
  docs: format kernel-parameters -- as code
  Doc : doc-guide : Fix a typo
  platform: x86: get rid of a non-existent document
  Add the RCU docs to the core-api manual
  Documentation: RCU: Add TOC tree hooks
  Documentation: RCU: Rename txt files to rst
  Documentation: RCU: Convert RCU UP systems to reST
  Documentation: RCU: Convert RCU linked list to reST
  Documentation: RCU: Convert RCU basic concepts to reST
  docs: filesystems: Remove uneeded .rst extension on toctables
  scripts/sphinx-pre-install: fix out-of-tree build
  docs: zh_CN: submitting-drivers.rst: Remove a duplicated Documentation/
  Documentation: PGP: update for newer HW devices
  Documentation: Add section about CPU vulnerabilities for Spectre
  Documentation: platform: Delete x86-laptop-drivers.txt
  docs: Note that :c:func: should no longer be used
  ...
2019-07-09 12:34:26 -07:00

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================
ARM64 ELF hwcaps
================
This document describes the usage and semantics of the arm64 ELF hwcaps.
1. Introduction
---------------
Some hardware or software features are only available on some CPU
implementations, and/or with certain kernel configurations, but have no
architected discovery mechanism available to userspace code at EL0. The
kernel exposes the presence of these features to userspace through a set
of flags called hwcaps, exposed in the auxilliary vector.
Userspace software can test for features by acquiring the AT_HWCAP or
AT_HWCAP2 entry of the auxiliary vector, and testing whether the relevant
flags are set, e.g.::
bool floating_point_is_present(void)
{
unsigned long hwcaps = getauxval(AT_HWCAP);
if (hwcaps & HWCAP_FP)
return true;
return false;
}
Where software relies on a feature described by a hwcap, it should check
the relevant hwcap flag to verify that the feature is present before
attempting to make use of the feature.
Features cannot be probed reliably through other means. When a feature
is not available, attempting to use it may result in unpredictable
behaviour, and is not guaranteed to result in any reliable indication
that the feature is unavailable, such as a SIGILL.
2. Interpretation of hwcaps
---------------------------
The majority of hwcaps are intended to indicate the presence of features
which are described by architected ID registers inaccessible to
userspace code at EL0. These hwcaps are defined in terms of ID register
fields, and should be interpreted with reference to the definition of
these fields in the ARM Architecture Reference Manual (ARM ARM).
Such hwcaps are described below in the form::
Functionality implied by idreg.field == val.
Such hwcaps indicate the availability of functionality that the ARM ARM
defines as being present when idreg.field has value val, but do not
indicate that idreg.field is precisely equal to val, nor do they
indicate the absence of functionality implied by other values of
idreg.field.
Other hwcaps may indicate the presence of features which cannot be
described by ID registers alone. These may be described without
reference to ID registers, and may refer to other documentation.
3. The hwcaps exposed in AT_HWCAP
---------------------------------
HWCAP_FP
Functionality implied by ID_AA64PFR0_EL1.FP == 0b0000.
HWCAP_ASIMD
Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0000.
HWCAP_EVTSTRM
The generic timer is configured to generate events at a frequency of
approximately 100KHz.
HWCAP_AES
Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0001.
HWCAP_PMULL
Functionality implied by ID_AA64ISAR0_EL1.AES == 0b0010.
HWCAP_SHA1
Functionality implied by ID_AA64ISAR0_EL1.SHA1 == 0b0001.
HWCAP_SHA2
Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0001.
HWCAP_CRC32
Functionality implied by ID_AA64ISAR0_EL1.CRC32 == 0b0001.
HWCAP_ATOMICS
Functionality implied by ID_AA64ISAR0_EL1.Atomic == 0b0010.
HWCAP_FPHP
Functionality implied by ID_AA64PFR0_EL1.FP == 0b0001.
HWCAP_ASIMDHP
Functionality implied by ID_AA64PFR0_EL1.AdvSIMD == 0b0001.
HWCAP_CPUID
EL0 access to certain ID registers is available, to the extent
described by Documentation/arm64/cpu-feature-registers.rst.
These ID registers may imply the availability of features.
HWCAP_ASIMDRDM
Functionality implied by ID_AA64ISAR0_EL1.RDM == 0b0001.
HWCAP_JSCVT
Functionality implied by ID_AA64ISAR1_EL1.JSCVT == 0b0001.
HWCAP_FCMA
Functionality implied by ID_AA64ISAR1_EL1.FCMA == 0b0001.
HWCAP_LRCPC
Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0001.
HWCAP_DCPOP
Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0001.
HWCAP2_DCPODP
Functionality implied by ID_AA64ISAR1_EL1.DPB == 0b0010.
HWCAP_SHA3
Functionality implied by ID_AA64ISAR0_EL1.SHA3 == 0b0001.
HWCAP_SM3
Functionality implied by ID_AA64ISAR0_EL1.SM3 == 0b0001.
HWCAP_SM4
Functionality implied by ID_AA64ISAR0_EL1.SM4 == 0b0001.
HWCAP_ASIMDDP
Functionality implied by ID_AA64ISAR0_EL1.DP == 0b0001.
HWCAP_SHA512
Functionality implied by ID_AA64ISAR0_EL1.SHA2 == 0b0010.
HWCAP_SVE
Functionality implied by ID_AA64PFR0_EL1.SVE == 0b0001.
HWCAP2_SVE2
Functionality implied by ID_AA64ZFR0_EL1.SVEVer == 0b0001.
HWCAP2_SVEAES
Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0001.
HWCAP2_SVEPMULL
Functionality implied by ID_AA64ZFR0_EL1.AES == 0b0010.
HWCAP2_SVEBITPERM
Functionality implied by ID_AA64ZFR0_EL1.BitPerm == 0b0001.
HWCAP2_SVESHA3
Functionality implied by ID_AA64ZFR0_EL1.SHA3 == 0b0001.
HWCAP2_SVESM4
Functionality implied by ID_AA64ZFR0_EL1.SM4 == 0b0001.
HWCAP_ASIMDFHM
Functionality implied by ID_AA64ISAR0_EL1.FHM == 0b0001.
HWCAP_DIT
Functionality implied by ID_AA64PFR0_EL1.DIT == 0b0001.
HWCAP_USCAT
Functionality implied by ID_AA64MMFR2_EL1.AT == 0b0001.
HWCAP_ILRCPC
Functionality implied by ID_AA64ISAR1_EL1.LRCPC == 0b0010.
HWCAP_FLAGM
Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
HWCAP2_FLAGM2
Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0010.
HWCAP_SSBS
Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.
HWCAP_PACA
Functionality implied by ID_AA64ISAR1_EL1.APA == 0b0001 or
ID_AA64ISAR1_EL1.API == 0b0001, as described by
Documentation/arm64/pointer-authentication.rst.
HWCAP_PACG
Functionality implied by ID_AA64ISAR1_EL1.GPA == 0b0001 or
ID_AA64ISAR1_EL1.GPI == 0b0001, as described by
Documentation/arm64/pointer-authentication.rst.
HWCAP2_FRINT
Functionality implied by ID_AA64ISAR1_EL1.FRINTTS == 0b0001.
4. Unused AT_HWCAP bits
-----------------------
For interoperation with userspace, the kernel guarantees that bits 62
and 63 of AT_HWCAP will always be returned as 0.
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