linux-kernel - Re: [PATCH v7 13/17] x86/asm: use fallback for random_get

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Date:   Mon, 02 May 2022 11:40:28 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        bp@...en8.de
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org
Subject: Re: [PATCH v7 13/17] x86/asm: use fallback for random_get_entropy()
 instead of zero

On Tue, Apr 26 2022 at 10:33, Jason A. Donenfeld wrote:

Subject: x86/tsc: Use .....

This has absolutely nothing to do with ASM and the sentence after the
colon starts with an uppercase letter.

> In the event that random_get_entropy() can't access a cycle counter or
> similar, falling back to returning 0 is suboptimal. Instead, fallback
> to calling random_get_entropy_fallback(), which isn't extremely high
> precision or guaranteed to be entropic, but is certainly better than
> returning zero all the time.
>
> If CONFIG_X86_TSC=n, then it's possible for the kernel to run on systems
> without RDTSC, such as 486 and certain 586, so the fallback code is only
> required for that case.
>
> As well, fix up both the new function and the get_cycles() function from
> which it was derived to use cpu_feature_enabled() rather than
> boot_cpu_has(), and use !IS_ENABLED() instead of #ifndef.
>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: x86@...nel.org

Same comments vs. Cc's

> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
> Changes v6->v7:
> - Adjust commit subject and body to match tip commit style.

Mostly ... :)

With that fixed:

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>