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

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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

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