linux-kernel - Re: random: Providing a seed value to VM guests

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Message-ID: <5362D20F.6010401@zytor.com>
Date:	Thu, 01 May 2014 16:00:31 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>,
	Florian Weimer <fweimer@...hat.com>,
	Kees Cook <kees@...flux.net>
Subject: Re: random: Providing a seed value to VM guests

On 05/01/2014 03:56 PM, Andy Lutomirski wrote:
> 
> I think we're comparing:
> 
> a) cpuid to detect rdrand *or* emulated rdrand followed by rdrand
> 
> to
> 
> b) cpuid to detect rdrand or the paravirt seed msr/cpuid call,
> followed by rdrand or the msr or cpuid read
> 
> this seems like it barely makes a difference, especially since (a)
> probably requires detecting KVM anyway.

Well, it lets one do something like:

	if (boot_cpu_has(X86_FEATURE_RDRAND) ||
            boot_cpu_has(X86_FEATURE_RDRAND_SIMULATED))
		rdrand_long(...);

We need the ifs anyway for early code; the arch_*() interfaces are only
available after alternatives run.

	-hpa

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