On 03/05/18 12:12, Kalra, Ashish wrote:
>
>>>
>>> The reason for the problem you're seeing is probably that the
>>> constraints are not quite right. Here, x0 is not simply clobbered, but
>>> is actively written to in the middle of the sequence (it is at least an
>>> early clobber). It is also, I assume, a result from the hypercall, so it
>>> cannot simply be discarded.
>>>
>>> It would help to get a disassembly of the function, but I'd tend to
>>> rewrite the code as such:
>>>
>>> extern int bar(void);
>>>
>>> int foo(void)
>>> {
>>> register int w0 asm("w0");
>>> int a, b;
>>>
>>> a = bar();
>>> w0 = 0x4b000000;
>>> asm volatile("hvc #0" : "+r" (w0) :: );
>>> b = bar();
>>>
>>> return a - b;
>>> }
>
> There still is the issue of x1-x3 registers being used for local
> variables and clobbering of the same across the "hvc" call.
>
> Comparatively, the fix below works more reliably :
>
> asm volatile("hvc #0" : "+r" (w0) :: "x1","x2","x3");
Is that because this hypercall is explicitly clobbering these registers?
or because KVM is corrupting them? If the former, fine. If the latter,
that'd be a KVM bug.
So which one is it?
M.
--
Jazz is not dead. It just smells funny...
