arm64: alternative: make alternative_has_cap_likely() VDSO compatible

Currently alternative_has_cap_unlikely() can be used in VDSO code, but
alternative_has_cap_likely() cannot as it references alt_cb_patch_nops,
which is not available when linking the VDSO. This is unfortunate as it
would be useful to have alternative_has_cap_likely() available in VDSO
code.

The use of alt_cb_patch_nops was added in commit:

  d926079 ("arm64: alternatives: add shared NOP callback")

... as removing duplicate NOPs within the kernel Image saved areasonable
amount of space.

Given the VDSO code will have nowhere near as many alternative branches
as the main kernel image, this isn't much of a concern, and a few extra
nops isn't a massive problem.

Change alternative_has_cap_likely() to only use alt_cb_patch_nops for
the main kernel image, and allow duplicate NOPs in VDSO code.

Signed-off-by: Mark Rutland <[email protected]>
Signed-off-by: Adhemerval Zanella <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>

Author: mrutland-arm

arch/arm64/include/asm/alternative-macros.h

@@ -230,7 +230,11 @@ alternative_has_cap_likely(const unsigned long cpucap)
 		return false;
 
 	asm goto(
+#ifdef BUILD_VDSO
+	ALTERNATIVE("b	%l[l_no]", "nop", %[cpucap])
+#else
 	ALTERNATIVE_CB("b	%l[l_no]", %[cpucap], alt_cb_patch_nops)
+#endif
 	:
 	: [cpucap] "i" (cpucap)
 	: