Re: random: Providing a seed value to VM guests

[H. Peter Anvin]

The normal CPUID bit is unset I believe.

On May 1, 2014 12:02:49 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On Thu, May 1, 2014 at 11:59 AM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> On 05/01/2014 11:53 AM, Andy Lutomirski wrote:
>>>
>>> A CPUID leaf or an MSR advertised by a CPUID leaf has another
>>> advantage: it's easy to use in the ASLR code -- I don't think
>there's
>>> a real IDT, so there's nothing like rdmsr_safe available.  It also
>>> avoids doing anything complicated with the boot process to allow the
>>> same seed to be used for ASLR and random.c; it can just be invoked
>>> twice on boot.
>>>
>>
>> At that point we are talking an x86-specific interface, and so we
>might
>> as well simply emulate RDRAND (urandom) and RDSEED (random) if the
>CPU
>> doesn't support them.  I believe KVM already has a way to report
>CPUID
>> features that are "emulated but supported anyway", i.e. they work but
>> are slow.
>
>Do existing kernels and userspace respect this?  If the normal bit for
>RDRAND is unset, then we might be okay, but, if not, then I think this
>may kill guest performance.
>
>Is RDSEED really reasonable here?  Won't it slow down by several
>orders of magnitude?
>
>>
>>> What's the right forum for this?  This thread is probably not it.
>>
>> Change the subject line?
>
>:)
>
>>
>>         -hpa
>>
>>

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