Re: [crash, bisected] Re: [PATCH 3/4] x86_64: Fold pda into per cpuarea

[Jeremy Fitzhardinge]

Eric W. Biederman wrote:
> Jeremy Fitzhardinge <jeremy@xxxxxxxx> writes:
>
>   
> > H. Peter Anvin wrote:
> >     
> > > Eric W. Biederman wrote:
> > >       
> > > > > The zero-based PDA mechanism requires the introduction of a new ELF segment
> > > > > based at vaddr 0 which is sufficiently unusual that it wouldn't surprise me
> > > > > if
> > > > > its triggering some toolchain bug.
> > > > >           
> > > > Agreed.  Given the previous description my hunch is that the bug is occurring
> > > > during objcopy.  If vmlinux is good and the compressed kernel is bad.
> > > >
> > > >         
> > > Actually, it's not all that unusual... it's pretty common in various
> > > restricted environments.  That being said, it's probably uncommon for *64-bit*
> > > code.
> > >       
> > Well, it's also unusual because 1) it's vaddr 0, but paddr <high>, and 2) the
> > PHDRs are not sorted by vaddr order.  2) might actually be a bug.
> >     
>
> I just looked and gcc does not use this technique for thread local data.
>   

Which technique?  It does assume you put the thread-local data near %gs 
(%fs in userspace), and it uses a small offset (positive or negative) to 
reach it.

At present, the x86-64 only uses %gs-relative addressing to reach the 
pda, which are always small positive offsets.  It always accesses 
per-cpu data in a two-step process of getting the base of per-cpu data, 
then offsetting to find the particular variable.

x86-32 has no pda, and arranges %fs so that %fs:variable gets the percpu 
variant of variable.  The offsets are always quite large.

> My initial concern about all of this was not making symbols section relative
> is relieved as this all appears to be a 64bit arch thing where that doesn't
> matter.
>   

Why's that?  I thought you cared particularly about making the x86-64 
kernel relocatable for kdump, and that using non-absolute symbols was 
part of that?

> Has anyone investigated using the technique gcc uses for thread local storage?
> http://people.redhat.com/drepper/tls.pdf
>   

The powerpc guys tried using gcc-level thread-local storage, but it 
doesn't work well.  per-cpu data and per-thread data have different 
constraints, and its hard to tell gcc about them.  For example, if you 
have a section of preemptable code in your function, it's hard to tell 
gcc not to cache a "thread-local" variable across it, even though we 
could have switched CPUs in the meantime.

> In particular using the local exec model so we can say:
> movq %fs:x@tpoff,%rax
>
> To load the contents of a per cpu variable x into %rax ?
>
> If we can use that model it should make it easier to interface with things like
> the stack protector code.  Although we would still need to be very careful
> about thread switches.
>   

You mean cpu switches?  We don't really have a notion of thread-local 
data in the kernel, other than things hanging off the kernel stack.

   J
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/