Messages in this thread | | | Date | Fri, 10 Nov 2006 13:46:53 +0100 | From | "Martin Schwidefsky" <> | Subject | Re: [kvm-devel] [PATCH] KVM: Avoid using vmx instruction directly |
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On 11/10/06, Jeremy Fitzhardinge <jeremy@goop.org> 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.
The "memory" clobber just tells the compiler that any memory object might get access by the inline. This forces the compiler to write back values it cached in registers and to reload the values after the inline assembly. This does NOT make it generate correct code for local objects. We had the case where we created a control block on the stack and passed it to a magic instruction. Since we did not tell the compiler that the content of the control block is used but only the address of it, gcc just passed a local stack address to the inline but optimized the initialization of the control block away. So the following can break:
struct control_block { int a, b; };
void fn(void) { struct control_block x;
x.a = 42; x.b = 0815; asm volatile ("<magic>" : : "a" (&x) : "memory"); }
You won't find the assignments to x.a and x.b in the compiled code.
-- blue skies, Martin - 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/
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