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vdso.7: srcfix: wrap some long source lines 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk 2014-01-02 08:54:51 +13:00
parent f6816de988
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@ -5,10 +5,10 @@
.\" %%%LICENSE_END
.\"
.\" Useful background:
.\" http://articles.manugarg.com/systemcallinlinux2_6.html
.\" https://lwn.net/Articles/446528/
.\" http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken
.\" http://www.trilithium.com/johan/2005/08/linux-gate/
.\" http://articles.manugarg.com/systemcallinlinux2_6.html
.\" https://lwn.net/Articles/446528/
.\" http://www.linuxjournal.com/content/creating-vdso-colonels-other-chicken
.\" http://www.trilithium.com/johan/2005/08/linux-gate/
.\"
.TH VDSO 7 2014-01-01 "Linux" "Linux Programmer's Manual"
.SH NAME
@ -18,7 +18,8 @@ vDSO \- overview of the virtual ELF dynamic shared object
.B void *vdso = (uintptr_t) getauxval(AT_SYSINFO_EHDR);
.SH DESCRIPTION
The "vDSO" is a small shared library that the kernel automatically maps into the
The "vDSO" is a small shared library that
the kernel automatically maps into the
address space of all user-space applications.
Applications usually do not need to concern themselves with these details
as the vDSO is most commonly called by the C library.
@ -27,14 +28,15 @@ and the C library will take care
of using any functionality that is available via the vDSO.
Why does the vDSO exist at all?
There are some system calls the kernel provides that user space ends up using
frequently, to the point that such calls can dominate overall performance.
There are some system calls the kernel provides that
user space code ends up using frequently,
to the point that such calls can dominate overall performance.
This is due both to the frequency of the call as well as the
context-switch overhead that results from
from exiting user space and entering the kernel.
The rest of this documentation is geared toward the curious and/or C library
writers rather than general developers.
The rest of this documentation is geared toward the curious and/or
C library writers rather than general developers.
If you're trying to call the vDSO in your own application rather than using
the C library, you're most likely doing it wrong.
.SS Example background
@ -48,7 +50,7 @@ in the processor's microcode as well as in the kernel.
Newer processors have faster (but backward incompatible) instructions to
initiate system calls.
Rather than require the C library to figure out if this functionality is
available at runtime,
available at run time,
the C library can use functions provided by the kernel in
the vDSO.
@ -56,27 +58,27 @@ Note that the terminology can be confusing.
On x86 systems, the vDSO function
used to determine the preferred method of making a system call is
named "__kernel_vsyscall", but on x86_64,
the term "vsyscall" also refers to an obsolete way to ask the kernel what time
it is or what CPU the caller is on.
the term "vsyscall" also refers to an obsolete way to ask the kernel
what time it is or what CPU the caller is on.
One frequently used system call is
.BR gettimeofday (2).
This system call is called both directly by user-space applications
as well as indirectly by
the C library.
Think timestamps or timing loops or polling\(emall of these frequently need to
know what time it is right now.
This information is also not secret\(emany application in any privilege mode
(root or any unprivileged user) will get the same answer.
Thus the kernel arranges for the information required to answer this question
to be placed in memory the process can access.
Think timestamps or timing loops or polling\(emall of these
frequently need to know what time it is right now.
This information is also not secret\(emany application in any
privilege mode (root or any unprivileged user) will get the same answer.
Thus the kernel arranges for the information required to answer
this question to be placed in memory the process can access.
Now a call to
.BR gettimeofday (2)
changes from a system call to a normal function
call and a few memory accesses.
.SS Finding the vDSO
The base address of the vDSO (if one exists) is passed by the kernel to each
program in the initial auxiliary vector (see
The base address of the vDSO (if one exists) is passed by the kernel to
each program in the initial auxiliary vector (see
.BR getauxval (3)),
via the
.B AT_SYSINFO_EHDR
@ -84,7 +86,7 @@ tag.
You must not assume the vDSO is mapped at any particular location in the
user's memory map.
The base address will usually be randomized at runtime every time a new
The base address will usually be randomized at run time every time a new
process image is created (at
.BR execve (2)
time).
@ -101,9 +103,9 @@ This tag is a throwback to the initial vDSO work (see
below) and its use should be avoided.
.SS File format
Since the vDSO is a fully formed ELF image, you can do symbol lookups on it.
This allows new symbols to be added with newer kernel releases, and allows the
C library to detect available functionality at runtime when running under
different kernel versions.
This allows new symbols to be added with newer kernel releases,
and allows the C library to detect available functionality at
run time when running under different kernel versions.
Oftentimes the C library will do detection with the first call and then
cache the result for subsequent calls.
@ -126,8 +128,8 @@ any of these functions.
No need to worry about weird register or stack behavior.
.SH NOTES
.SS Source
When you compile the kernel, it will automatically compile and link the vDSO
code for you.
When you compile the kernel,
it will automatically compile and link the vDSO code for you.
You will frequently find it under the architecture-specific directory:
find arch/$ARCH/ -name '*vdso*.so*' -o -name '*gate*.so*'
@ -206,12 +208,13 @@ __kernel_clock_getres LINUX_2.6.39
.SS bfin (Blackfin) functions
.\" See linux/arch/blackfin/kernel/fixed_code.S
.\" See http://docs.blackfin.uclinux.org/doku.php?id=linux-kernel:fixed-code
As this CPU lacks a memory management unit (MMU), it doesn't set up a vDSO in
the normal sense.
Instead, it maps at boot time a few raw functions into a fixed location in
memory.
As this CPU lacks a memory management unit (MMU),
it doesn't set up a vDSO in the normal sense.
Instead, it maps at boot time a few raw functions into
a fixed location in memory.
User-space applications then call directly into that region.
There is no provision for backward compatibility beyond sniffing raw opcodes,
There is no provision for backward compatibility
beyond sniffing raw opcodes,
but as this is an embedded CPU, it can get away with things\(emsome of the
object formats it runs aren't even ELF based (they're bFLT/FLAT).
@ -240,8 +243,8 @@ __kernel_syscall_via_epc LINUX_2.5
\}
The Itanium port is somewhat tricky.
In addition to the vDSO above, it also has "light-weight system calls" (also
known as "fast syscalls" or "fsys").
In addition to the vDSO above, it also has "light-weight system calls"
(also known as "fast syscalls" or "fsys").
You can invoke these via the
.I __kernel_syscall_via_epc
vDSO helper.
@ -272,7 +275,8 @@ set_tid_address
.SS parisc (hppa) functions
.\" See linux/arch/parisc/kernel/syscall.S
.\" See linux/Documentation/parisc/registers
The parisc port has a code page full of utility functions called a gateway page.
The parisc port has a code page full of utility functions
called a gateway page.
Rather than use the normal ELF auxiliary vector approach,
it passes the address of
the page to the process via the SR2 register.