Re: [PATCH 1/2] MIPS: Preliminary vdso.

[David Daney <ddaney@caviumnetworks.com>]

Kevin D. Kissell wrote:
> David Daney wrote:
>> This is a preliminary patch to add a vdso to all user processes.
>> Still missing are ELF headers and .eh_frame information.  But it is
>> enough to allow us to move signal trampolines off of the stack.
>>
>> We allocate a single page (the vdso) and write all possible signal
>> trampolines into it.  The stack is moved down by one page and the vdso
>> is mapped into this space.
>>
>> Signed-off-by: David Daney <ddaney@caviumnetworks.com>
> Note that for FPU-less CPUs, the kernel FP emulator also uses a user
> stack trampoline to execute instructions in the delay slots of emulated
> FP branches.  I didn't see any of the math-emu modules being tweaked in
> either part of your patch.  Presumably, one would want to move that
> operation into the vdso as well.  With the proposed patch, I'm not sure
> whether things would continue working normally as before, still using
> the user stack, or whether the dsemulret code depends on something that
> is changed by the patch, and will now implode.  Probably the former, but
> paranoia is not a character defect in OS kernel work. I don't have a
> test case handy (nor a test system any more), but compiling something
> like whetstone or linpack in C with a high degree of optimization will
> *probably* generate FP branches with live delay slots.
> 

It is an ugly problem.  I am trying to hack something up to fix it.

David Daney