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SubjectRe: [PATCH v6 04/12] x86/alternative: support not-feature
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On 10.03.21 07:07, Borislav Petkov wrote:
> On Tue, Mar 09, 2021 at 02:48:05PM +0100, Juergen Gross wrote:
>> 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))
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V5:
>> - split off from next patch
>> - reworked to use flag byte (Boris Petkov)
>> V6:
>> - rework again to not use flag byte (Boris Petkov)
>> ---
>> arch/x86/include/asm/alternative-asm.h | 3 +++
>> arch/x86/include/asm/alternative.h | 3 +++
>> arch/x86/kernel/alternative.c | 19 ++++++++++++++-----
>> 3 files changed, 20 insertions(+), 5 deletions(-)
>
> LGTM, minor touchups I'd do ontop:
>
> ---
>
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index 89889618ae01..fd205cdcfbad 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -55,18 +55,18 @@
> ".long 999b - .\n\t" \
> ".popsection\n\t"
>
> +#define ALTINSTR_FLAG_INV (1 << 15)
> +#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
> +
> struct alt_instr {
> s32 instr_offset; /* original instruction */
> s32 repl_offset; /* offset to replacement instruction */
> u16 cpuid; /* cpuid bit set for replacement */
> -#define ALTINSTR_FLAG_INV (1 << 15)
> u8 instrlen; /* length of original instruction */
> u8 replacementlen; /* length of new instruction */
> u8 padlen; /* length of build-time padding */
> } __packed;
>
> -#define ALT_NOT(feat) ((feat) | ALTINSTR_FLAG_INV)
> -

Did you look at patch 13? :-)

> /*
> * Debug flag that can be tested to see whether alternative
> * instructions were patched in already:
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index d8e669a1546f..133b549dc091 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -397,9 +397,10 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
> BUG_ON(feature >= (NCAPINTS + NBUGINTS) * 32);
>
> /*
> - * Drop out if either:
> - * - feature not available, but required, or
> - * - feature available, but NOT required
> + * Patch if either:
> + * - feature is present
> + * - feature not present but ALTINSTR_FLAG_INV is set to mean,
> + * patch if feature is *NOT* present.

Okay.


Juergen
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