[RFC] arm: vdso: Convert sigpage to vdso implementation

[Nathan_Lynch@mentor.com (Nathan Lynch)]

Hi Steve,

On 01/28/2014 10:25 AM, Steve Capper wrote:
> ARM has a special sigpage that is used for signal return trampolines.
> Its implementation is very similar to a VDSO conceptually in that it
> occupies a special mapping in user address space.
> 
> One could actually host the trampoline code in a VDSO instead with the
> added advantage that one could also host specialised routines there.
> One such routine could be gettimeofday where on ARM we have architected
> (and some vendor supplied) timers that can be queried entirely in
> userspace, obviating the need for an expensive syscall.
> 
> This patch converts the sigpage implementation to a VDSO. It is mostly
> a direct port from Will Deacon's arm64 implementation with the ARM
> signal trampoline plumbed in.
> 
> Signed-off-by: Steve Capper <steve.capper@linaro.org>
> ---
> As can be inferred from this RFC, I am interested ultimately in
> implementing a syscall-less gettimeofday for ARM. Whilst researching
> possible vectors page or VDSO implementations, I came across the
> sigpage mechanism which is very similar to a VDSO.
> 
> The very simple function, __kernel_vdso_doubler, resolved in a test
> program automatically on my Arndale board (running Fedora 20) without
> any additional prodding.
> 
> IPC stress tests from LTP were executed to test the signal trampoline.
> 
> I would appreciate any comments on this approach of converting the
> sigpage to a VDSO. If this looks sane to people, I will work on the
> gettimeofday logic in a later patch.

As it happens, I've been working on a vDSO implementation of
gettimeofday/clock_gettime which does not mess with the signal page.
I'll reply with the patch separately in a moment.