Re: [PATCH v6 04/12] x86/alternative: support not-feature

[Jürgen Groß <jgross@xxxxxxxx>]

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@xxxxxxxx>
> > ---
> > 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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