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From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        arnd@arndb.de, Borislav Petkov <bp@alien8.de>, x86@kernel.org
Subject: Re: [PATCH v6 13/17] x86: use fallback for random_get_entropy()
 instead of zero
In-Reply-To: <Ymak7LJd6GnFxsOo@zx2c4.com>
References: <20220423212623.1957011-1-Jason@zx2c4.com>
 <20220423212623.1957011-14-Jason@zx2c4.com> <871qxl2vdw.ffs@tglx>
 <Ymak7LJd6GnFxsOo@zx2c4.com>
Date:   Mon, 25 Apr 2022 19:03:54 +0200
Message-ID: <87ilqx14ed.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On Mon, Apr 25 2022 at 15:41, Jason A. Donenfeld wrote:
> On Mon, Apr 25, 2022 at 02:35:39PM +0200, Thomas Gleixner wrote:
>> > +static inline unsigned long random_get_entropy(void)
>> > +{
>> > +#ifndef CONFIG_X86_TSC
>> > +	if (!cpu_feature_enabled(X86_FEATURE_TSC))
>> > +		return random_get_entropy_fallback();
>> > +#endif
>> 
>> Please get rid of this ifdeffery. While you are right, that anything
>> with CONFIG_X86_TSC=y should have a TSC, there is virt ....
>> 
>> cpu_feature_enabled() is runtime patched and only evaluated before
>> alternative patching, so the win of this ifdef is marginally, if even
>> noticable.
>> 
>> We surely can think about making TSC mandatory, but not selectively in a
>> particalur context.
>
> This would be a regression of sorts from the current code, which reads:
>
>     static inline cycles_t get_cycles(void)
>     {
>     #ifndef CONFIG_X86_TSC
>             if (!boot_cpu_has(X86_FEATURE_TSC))
>                     return 0;
>     #endif
>             return rdtsc();
>     }

Bah. Indeed. Misread the patch, but yes, if you are at it to make that
cpu_feature_enabled() then change the config thing to IS_ENABLED() too.

Thanks,

        tglx