RDSEED is not synchronous. It is, however, nonblocking. On May 1, 2014 1:16:40 PM PDT, Andy Lutomirski <l...@amacapital.net> wrote: >On May 1, 2014 12:26 PM, <ty...@mit.edu> 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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