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

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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
This commit is contained in:
H. Peter Anvin
2021-09-10 12:59:10 -07:00
committed by Borislav Petkov
parent f87bc8dc7a
commit 0507503671
1 changed files with 9 additions and 4 deletions
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arch/x86/include/asm/cpufeature.h
+9 -4
View File
@@ -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;