Hi all- I would like to standardize on a very simple protocol by which a guest OS can obtain an RNG seed early in boot. The main design requirements are: - The interface should be very easy to use. Linux, at least, will want to use it extremely early in boot as part of kernel ASLR. This means that PCI and ACPI will not work. - It should be synchronous. We don't want to delay boot while waiting for a slow host RNG. (On Linux, at least, we have a separate interface for that: virtio-rng. I think that Windows has some support for virtio-rng as well.) - Random numbers obtained through this interface should be best-effort. We want the best quality randomness that the host can provide immediately. It seems to me that the best interface for the actual request for a random number is rdmsr. This is supported on all hypervisors and all virtualization technologies. It can return a 64 bit random number, and it is easy to rdmsr the same register more than once to get a larger random number. The main questions are what MSR index to use and how to detect the presence of the MSR. I've played with two approaches: 1. Use CPUID to detect the presence of this feature. This is very easy for KVM to implement by using a KVM-specific CPUID feature. The problem is that this will necessarily be KVM-specific, as the guest must first probe for KVM and then probe for the KVM feature. I doubt that Hyper-V, for example, wants to claim to be KVM. If we could standardize a non-hypervisor-specific CPUID feature, then this problem would go away. 2. Detect the existence of the MSR by trying to read it and handling the #GP(0) that will occur if the MSR is not present. Linux, at least, is okay with doing this, and I have code to enable an IDT and an rdmsr fixup early enough in boot to use it for ASLR. I don't know whether other operating systems can do this, though. The major questions, then, are what enumeration mechanism should be used and what MSR index should be used. For the MSR index, we could use an MSR from the Intel range if Intel were to give explicit approval, thus guaranteeing that nothing would conflict. Or we could try to agree on an MSR index in the 0x40000000-0x4fffffff range that is unlikely to conflict with anything. For enumeration, we could just probe the MSR if all relevant guests are okay with this or we could standardize on a CPUID-based mechanism. If we do the latter, I don't know what that mechanism would be. NB: This thread will be cc'd to Microsoft and possibly Hyper-V people shortly. I very much appreciate Jun Nakajima's help with this! Thanks, Andy -- Andy Lutomirski AMA Capital Management, LLC _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
