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

[Andi Kleen <ak@suse.de>]

On Thursday 22 June 2006 01:18, Rohit Seth wrote:
> On Thu, 2006-06-22 at 01:05 +0200, Andi Kleen wrote:
> > On Thursday 22 June 2006 00:59, Rohit Seth wrote:
> 
> > > 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).
> > 
> > I'm still not sure where and for what you want to use this. In user space 
> > or in kernel space? And what information should be stored in there?
> > 
> 
> Store the kernel virtual pointer in gdt to access pda in (proposed)
> vgetcpu in vsyscall. 
> Using this pointer we can easily reach the cpu and 
> node numbers and any other information that is there in pda.  For the
> cpu and node numbers this will get rid of the need to do a serializing
> operation cpuid.
> 
> Does it make any sense?

Ok to spell it out (please correct me if I misinterpreted you). You want to:

- Split PDA into kernel part and user exportable part
- Export user exportable part to ring 3
- Put base address of user exportable part into GDT
- Access it using that.

I don't think it can work because the GDT only supports 32bit
base addresses for code/data segments in long mode and you can't put
a kernel virtual address into 32bit (only user space there) 

And you can't get at at the base address anyways because they
are ignored in long mode (except for fs/gs). For fs/gs you would
need to save/restore them to reuse them which would be slow.

You can't also just put them into fs/gs because those are
already reserved for user space.

Also I don't know what other information other than cpu/node 
would be useful, so just using the 20 bits of limit seems plenty to me.

-Andi