From: Juergen Gross jgross@suse.com
commit dda7bb76484978316bb412a353789ebc5901de36 upstream.
Add support for alternative patching for the case a feature is not present on the current CPU. For users of ALTERNATIVE() and friends, an inverted feature is specified by applying the ALT_NOT() macro to it, e.g.:
ALTERNATIVE(old, new, ALT_NOT(feature));
Committer note:
The decision to encode the NOT-bit in the feature bit itself is because a future change which would make objtool generate such alternative calls, would keep the code in objtool itself fairly simple.
Also, this allows for the alternative macros to support the NOT feature without having to change them.
Finally, the u16 cpuid member encoding the X86_FEATURE_ flags is not an ABI so if more bits are needed, cpuid itself can be enlarged or a flags field can be added to struct alt_instr after having considered the size growth in either cases.
Signed-off-by: Juergen Gross jgross@suse.com Signed-off-by: Borislav Petkov bp@suse.de Link: https://lkml.kernel.org/r/20210311142319.4723-6-jgross@suse.com Signed-off-by: Ben Hutchings ben@decadent.org.uk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/x86/include/asm/alternative.h | 3 +++ arch/x86/kernel/alternative.c | 20 +++++++++++++++----- 2 files changed, 18 insertions(+), 5 deletions(-)
--- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -6,6 +6,9 @@ #include <linux/stringify.h> #include <asm/asm.h>
+#define ALTINSTR_FLAG_INV (1 << 15) +#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV) + #ifndef __ASSEMBLY__
#include <linux/stddef.h> --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -388,21 +388,31 @@ void __init_or_module noinline apply_alt */ for (a = start; a < end; a++) { int insn_buff_sz = 0; + /* Mask away "NOT" flag bit for feature to test. */ + u16 feature = a->cpuid & ~ALTINSTR_FLAG_INV;
instr = (u8 *)&a->instr_offset + a->instr_offset; replacement = (u8 *)&a->repl_offset + a->repl_offset; BUG_ON(a->instrlen > sizeof(insn_buff)); - BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32); - if (!boot_cpu_has(a->cpuid)) { + BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32); + + /* + * Patch if either: + * - feature is present + * - feature not present but ALTINSTR_FLAG_INV is set to mean, + * patch if feature is *NOT* present. + */ + if (!boot_cpu_has(feature) == !(a->cpuid & ALTINSTR_FLAG_INV)) { if (a->padlen > 1) optimize_nops(a, instr);
continue; }
- DPRINTK("feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d", - a->cpuid >> 5, - a->cpuid & 0x1f, + DPRINTK("feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d), pad: %d", + (a->cpuid & ALTINSTR_FLAG_INV) ? "!" : "", + feature >> 5, + feature & 0x1f, instr, instr, a->instrlen, replacement, a->replacementlen, a->padlen);