linux-kernel - Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG

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Date:	Wed, 16 Jul 2014 13:20:53 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Gleb Natapov <gleb@...nel.org>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	"Theodore Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/4] random,x86,kvm: Add and use MSR_KVM_GET_RNG_SEED

On 07/16/2014 09:21 AM, Gleb Natapov wrote:
> On Wed, Jul 16, 2014 at 09:13:23AM -0700, H. Peter Anvin wrote:
>> On 07/16/2014 09:08 AM, Paolo Bonzini wrote:
>>> Il 16/07/2014 18:03, H. Peter Anvin ha scritto:
>>>> I suggested emulating RDRAND *but not set the CPUID bit*.  We already
>>>> developed a protocol in KVM/Qemu to enumerate emulated features (created
>>>> for MOVBE as I recall), specifically to service the semantic "feature X
>>>> will work but will be substantially slower than normal."
>>>
>>> But those will set the CPUID bit.  There is currently no way for KVM
>>> guests to know if a CPUID bit is real or emulated.
>>>
>>
>> OK, so there wasn't any protocol implemented in the end.  I sit corrected.
>>
> That protocol that was implemented is between qemu and kvm, not kvm and a guest.
> 

Either which way, the notion was to have a PV CPUID bit like the
proposed kvm_get_rng_seed bit, but to have it exercised by executing RDRAND.

The biggest reason to *not* do this would be that with an MSR it is not
available to guest user space, which may be better under the circumstances.

	-hpa


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