Re: [PATCH v5 07/11] x86: use fallback for random_get_entropy() instead of zero

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: Borislav Petkov <bp@alien8.de>
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Subject: Re: [PATCH v5 07/11] x86: use fallback for random_get_entropy() instead of zero
Date: Tue, 19 Apr 2022 20:38:41 +0200	[thread overview]
Message-ID: <CAHmME9q03Je-ROzzHCgZC0vy1n=y8bsGBOAs8U_K_r3ebLNHbw@mail.gmail.com> (raw)
In-Reply-To: <Yl78gLLcSb3EHv0B@zn.tnic>

Hi Borislav,

On Tue, Apr 19, 2022 at 8:16 PM Borislav Petkov <bp@alien8.de> wrote:
> > +static inline unsigned long random_get_entropy(void)
> > +{
> > +#ifndef CONFIG_X86_TSC
> > +     if (!boot_cpu_has(X86_FEATURE_TSC))
>
> cpu_feature_enabled() pls.

This function began as a carbon copy of get_cycles(), 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();
}

As you see, random_get_entropy() is the same function, but with that
zero replaced with the fallback. (Using the fallback in get_cycles()
wouldn't be appropriate.)

So, your options are:
a) We keep this patch as-is, using boot_cpu_has(); or
b) I make an unrelated change inside of this same patch to change
get_cycles() to use cpu_feature_enabled() (in addition to the new
random_get_entropy()).

I think I prefer doing (a), and leaving (b) for another time when you
or another x86 maintainer can do so. But I'll do whichever you say.
Which would you like?

Jason

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