Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly

[Avi Kivity <avi@qumranet.com>]

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
> > Avi Kivity wrote:
> > > > Or gcc
> > > > might move the assignment of phys_addr to after the inline assembly.
> > > >   
> > > "asm volatile" prevents that (and I'm not 100% sure it's necessary).
> >
> > No, it won't necessarily.  "asm volatile" simply forces gcc to emit the
> > assembler, even if it thinks its output doesn't get used.  It makes no
> > ordering guarantees with respect to other code (or even other "asm
> > volatiles").   The "memory" clobbers should fix the ordering of the asms
> > though.
>
> I think you're wrong about that; in particular, I'm pretty sure "asm 
> volatiles" are ordered among themselves.  What the "volatile" means is 
> "this has side effects you (the compiler) don't understand", and gcc 
> can't assume that it can reorder such side effects.

The gcc manual has this to say:

  Similarly, you can't expect a sequence of volatile `asm' instructions
 to remain perfectly consecutive.  If you want consecutive output, use a
 single `asm'.  Also, GCC will perform some optimizations across a
 volatile `asm' instruction; GCC does not "forget everything" when it
 encounters a volatile `asm' instruction the way some other compilers do.

I wonder how we are supposed to code the following sequence:


   asm volatile ("blah")  /* sets funky processor mode */

   some_c_code();

   asm volatile ("unblah");

Let's say "blah" disables floating point exceptions, and some_c_code() 
must run without exceptions.  Is is possible to code this in gcc without 
putting functions in another translation unit?  Is a memory clobber 
sufficient?  I'd certainly hate to use it.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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