The following commit has been merged into the x86/cpu branch of tip: Commit-ID: 0507503671f9b1c867e889cbec0f43abf904f23c Gitweb: https://git.kernel.org/tip/0507503671f9b1c867e889cbec0f43abf904f23c Author: H. Peter Anvin <hpa@xxxxxxxxx> AuthorDate: Fri, 10 Sep 2021 12:59:10 -07:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Mon, 13 Sep 2021 19:48:21 +02:00 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@xxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Link: https://lkml.kernel.org/r/20210910195910.2542662-4-hpa@xxxxxxxxx --- arch/x86/include/asm/cpufeature.h | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index 16a51e7..1261842 100644 --- 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;
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