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

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Date:   Wed, 10 Mar 2021 08:52:40 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v6 04/12] x86/alternative: support not-feature

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@...e.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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