objtool complains:
arch/x86/kvm/kvm.o: warning: objtool: .altinstr_replacement+0xc5: call without frame pointer save/setup
vmlinux.o: warning: objtool: .altinstr_replacement+0x2eb: call without frame pointer save/setup
Make sure %rSP is an output operand to the respective asm() statements.
The test_cc() hunk and ALT_OUTPUT_SP() courtesy of peterz. Also from him
add some helpful debugging info to the documentation.
Now on to the explanations:
tl;dr: The alternatives macros are pretty fragile.
If I do ALT_OUTPUT_SP(output) in order to be able to package in a %rsp
reference for objtool so that a stack frame gets properly generated, the
inline asm input operand with positional argument 0 in clear_page():
"0" (page)
gets "renumbered" due to the added
: "+r" (current_stack_pointer), "=D" (page)
and then gcc says:
./arch/x86/include/asm/page_64.h:53:9: error: inconsistent operand constraints in an ‘asm’
The fix is to use an explicit "D" constraint which points to a singleton
register class (gcc terminology) which ends up doing what is expected
here: the page pointer - input and output - should be in the same %rdi
register.
Other register classes have more than one register in them - example:
"r" and "=r" or "A":
‘A’
The ‘a’ and ‘d’ registers. This class is used for
instructions that return double word results in the ‘ax:dx’
register pair. Single word values will be allocated either in
‘ax’ or ‘dx’.
so using "D" and "=D" just works in this particular case.
And yes, one would say, sure, why don't you do "+D" but then:
: "+r" (current_stack_pointer), "+D" (page)
: [old] "i" (clear_page_orig), [new1] "i" (clear_page_rep), [new2] "i" (clear_page_erms),
: "cc", "memory", "rax", "rcx")
now find the Waldo^Wcomma which throws a wrench into all this.
Because that silly macro has an "input..." consume-all last macro arg
and in it, one is supposed to supply input *and* clobbers, leading to
silly syntax snafus.
Yap, they need to be cleaned up, one fine day...
Closes: https://lore.kernel.org/oe-kbuild-all/202406141648.jO9qNGLa-lkp@intel.com/
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Sean Christopherson <seanjc@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240625112056.GDZnqoGDXgYuWBDUwu@fat_crate.local
0d3db1f14a