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

Avi Kivity
2006-11-09T14:52:40Z

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