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

[Avi Kivity <avi@qumranet.com>]

Arnd Bergmann wrote:
> On Thursday 09 November 2006 14:36, Avi Kivity wrote:
>   
> > > I'm not an expert on inline assembly, but don't you need an extra
> > > '"m" (phys_addr)' to make sure that gcc actually puts the variable
> > > on the stack instead of passing a NULL pointer as '"a"(&phys_addr)'?
> > >       
> > Taking a variable's address should force its contents into memory (like 
> > calling an uninlined function with &var).
> >     
>
> No it doesn't. You're not telling gcc that the inline assembly cares
> about the contents of the variable, so it could be a reference to
> a stack slot while the contents are still in a register. 

Wouldn't that make inline assembly useless?  Suppose the contents is 
itself a pointer.  What about the pointed-to contents?

e.g.

   int x = 3;
   int *y = &x;
   int z;

   asm ("mov %1, %%rax; movl (%%rax), %0" : "=r"(z) : "g"(y) : "rax");
   assert(z == 3);

> 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).


--
error compiling committee.c: too many arguments to function

-
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/