Re: random: Providing a seed value to VM guests

[H. Peter Anvin]

RDSEED is not synchronous.  It is, however, nonblocking.

On May 1, 2014 1:16:40 PM PDT, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>On May 1, 2014 12:26 PM, <tytso@xxxxxxx> wrote:
>>
>> On Thu, May 01, 2014 at 12:02:49PM -0700, Andy Lutomirski wrote:
>> >
>> > Is RDSEED really reasonable here?  Won't it slow down by several
>> > orders of magnitude?
>>
>> That is I think the biggest problem; RDRAND and RDSEED are fast if
>> they are native, but they will involve a VM exit if they need to be
>> emulated.  So when an OS might want to use RDRAND and RDSEED might be
>> quite different if we know they are being emulated.
>>
>> Using the RDRAND and RDSEED "api" certainly makes sense, at least for
>> x86, but I suspect we might want to use a different way of signalling
>> that a VM guest can use RDRAND and RDSEED if they are running on a
>CPU
>> which doesn't provide that kind of access.  Maybe a CPUID extended
>> function parameter, if one could be allocated for use by a Linux
>> hypervisor?
>>
>
>I'm still not convinced.  This will affect userspace as well as the
>guest kernel, and I don't see why guest user code should be able to
>access this API.  RDRAND for CPL0 only would work, but that seems odd.
>
>And I think that RDSEED emulation is asking for trouble.  RDSEED is
>synchronous, but /dev/random is asynchronous.  And making bootup wait
>for even a single byte from /dev/random seems bad.  In any event,
>virtio-rng should be a better interface for this.
>
>>                                                 - Ted
>>

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