Re: [RFC, patch] i386: vgetcpu(), take 2

[Rohit Seth <rohitseth@google.com>]

On Thu, 2006-06-22 at 00:21 +0200, Andi Kleen wrote:
> > Can we use similar  mechanism to access pda in vsyscall in x86_64 (by
> > storing the address of pda there).  
> 
> 
> You mean in the kernel? %gs prefix is a lot faster than this.
> 

Yes it is.  And will work if we are okay to swap to kernel gs in
vsyscall code.

> Also the limit is only 20bit, not enough for a full address.
> 

I was thinking of storing it is base address part of the descriptor and
then using the memory load to read it in vsyscall.  (Keeping the p bit
to zero in the descriptor).

> For user space it's useful though, but I don't see any immediate uses
> other than cpu number and node number. For most purposes glibc TLS
> (which uses %fs) is probably sufficient.

cpu and node number are really important (for the reasons that you
mentioned in your initial mail on vgetcpu).  In addition to that I was
thinking in terms of having some counters like nmi_count that is already
there and per cpu specific.

Besides, not having to use the tcache part in the proposed system call
seems to just make the interface cleaner. 

-rohit