Booting dom0 and running xend spews quite a number of:
(XEN) (file=multicall.c, line=50) Error writing result back to multicall block.
To jump right to the end of a long story, i think that
do_multicall_call() is in x86_64 abi violation.
#define do_multicall_call(_call) \
do { \
__asm__ __volatile__ ( \
"movq "STR(MULTICALL_op)"(%0),%%rax; " \
...
"movq "STR(MULTICALL_arg4)"(%0),%%r8; " \
"callq *(%%rax); " \
"movq %%rax,"STR(MULTICALL_result)"(%0); " \
: : "b" (_call) \
: "rax", "rdi", "rsi", "rdx", "rcx", "r8"); \
} while ( 0 )
The abi says the following (v0.95, page 16):
Registers %rbp, %rbx and %r12 through %r15 "belong" to the
calling function and the called function is required to preserve
their values. In other words, a called function must preserve
these registers' values for its caller. Remaining registers
"belong" to the called function. If a calling function wants to
preserve such a register value across a function call, it must
save the value in its local stack frame.
do_multicall_call() is doing a call, who does who knows what with
registers, but doesn't advertise clobbering all the abi's volatile
registers.
I tried searching for documentation, as it seems to me that gcc should
have some magic flag since it has to know the abi anyway, but i couldn't
find anything. Assuming that this isn't a compiler bug and there aren't
any such flags, the attached patch should make things abi compliant, and
fixes the problem i was seeing.
-----
If you're curious the exact connection between the abi violation and
the error messages...
do_multicall() looks somewhat like (pseudo-code):
do_multicall(...) {
if ( asm("inline-asm-1") )
return;
do_multicall_call();
if ( asm("inline-asm-2") ) {
printk("Error");
return;
}
the result of inline-asm-1 is in some register X. the "if" turns into a
jnz on X to a "ret". gcc realizes if we didn't jump, then X has to be 0,
so it's going to use that value for initialization of the __pu_err in
inline-asm-2.
the problem is that the call in do_multicall_call() stomps on X, as it
isn't advertised to clobber X.
inline-asm-2 is the __put_user code, which uses the fixup/exception-
table trick:
"1: mov"itype" %"rtype"1,%2\n" \
"2:\n" \
".section .fixup,\"ax\"\n" \
"3: mov %3,%0\n" \
" jmp 2b\n" \
".previous\n" \
".section __ex_table,\"a\"\n" \
" "__FIXUP_ALIGN"\n" \
" "__FIXUP_WORD" 1b,3b\n" \
".previous" \
: "=r"(err) \
: ltype (x), "m"(__m(addr)), "i"(errret), "0"(err))
so "err" (in register X) is uninitialized if we don't fault. finally
depending on what the call did with X if could be a non-zero value.
sRp
--
Scott Parish
Signed-off-by: srparish@xxxxxxxxxx
register-ditty.diff
Description: Text document
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