Compat VDSO support ?

[Net Llama!]

On Mon, 27 Nov 2006, Matthew Carpenter wrote:
> On Friday 24 November 2006 15:22, Net Llama! wrote:
>> Anyone else here using a kernel without Compat VDSO support ?  For those
>> scratching their heads, it seems to be a new option in 2.6.18.x kernels:
>> Say N here if you are running a sufficiently recent glibc
>> version (2.3.3 or later), to remove the high-mapped
>> VDSO mapping and to exclusively use the randomized VDSO.
>>
>> If you have a kernel where this option was disabled, did you notice any
>> performance or functionality differences?
>
> VDSO is a piece of memory mapped into every running process, and holds the
> code used to talk to the kernel from userspace.  Older hardware relied
> completely on an Interrupt (which interrupt differs between POSIX OSes).
> Years ago, hardware vendors made a new opcode available, known as SYSENTER.
> SYSENTER has had a dodgy past, starting out neither reliable nor necessarily
> faster than the interrupts.  Over the past half a decade, SYSENTER has found
> stability and offers significant performance improvements.
>
> In order to make the decision a boot-time option, the appropriate binary code
> (int 0x80 or SYSENTER) is mapped into each process as this "VDSO" section.
> glibc calls which needs access to the kernel, rather than calling int 0x80 or
> sysenter themselves, rather jmp to this portion of process memory space and
> use whatever the kernel set up for you at boot time.

It doesn't neccesarily tell me which one is best for specific hardware, 
but its good info to know anyway.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Lonni J Friedman                        netllama at linux-sxs.org
LlamaLand				http://netllama.linux-sxs.org