x86/asm: Avoid adding register pressure for the init case in static_cpu_has()

gcc will sometimes manifest the address of boot_cpu_data in a register
as part of constant propagation. When multiple static_cpu_has() are used
this may foul the mainline code with a register load which will only be
used on the fallback path, which is unused after initialization.

Explicitly force gcc to use immediate (rip-relative) addressing for
the fallback path, thus removing any possible register use from
static_cpu_has().

While making changes, modernize the code to use
.pushsection...popsection instead of .section...previous.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210910195910.2542662-4-hpa@zytor.com

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 16a51e7..1261842 100644 (file)
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -173,20 +173,25 @@ extern void clear_cpu_cap(struct cpuinfo_x86 *c, unsigned int bit);
  * means that the boot_cpu_has() variant is already fast enough for the
  * majority of cases and you should stick to using it as it is generally
  * only two instructions: a RIP-relative MOV and a TEST.
+ *
+ * Do not use an "m" constraint for [cap_byte] here: gcc doesn't know
+ * that this is only used on a fallback path and will sometimes cause
+ * it to manifest the address of boot_cpu_data in a register, fouling
+ * the mainline (post-initialization) code.
  */
 static __always_inline bool _static_cpu_has(u16 bit)
 {
        asm_volatile_goto(
                ALTERNATIVE_TERNARY("jmp 6f", %P[feature], "", "jmp %l[t_no]")
-               ".section .altinstr_aux,\"ax\"\n"
+               ".pushsection .altinstr_aux,\"ax\"\n"
                "6:\n"
-               " testb %[bitnum],%[cap_byte]\n"
+               " testb %[bitnum]," _ASM_RIP(%P[cap_byte]) "\n"
                " jnz %l[t_yes]\n"
                " jmp %l[t_no]\n"
-               ".previous\n"
+               ".popsection\n"
                 : : [feature]  "i" (bit),
                     [bitnum]   "i" (1 << (bit & 7)),
-                    [cap_byte] "m" (((const char *)boot_cpu_data.x86_capability)[bit >> 3])
+                    [cap_byte] "i" (&((const char *)boot_cpu_data.x86_capability)[bit >> 3])
                 : : t_yes, t_no);
 t_yes:
        return true;