On Thu, May 1, 2014 at 1:30 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: > RDSEED is not synchronous. It is, however, nonblocking. What I mean is: IIUC it's reasonable to call RDSEED a few times in a loop and hope it works. It makes no sense to do that with /dev/random. > > 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 >>> > > -- > Sent from my mobile phone. Please pardon brevity and lack of formatting. -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
