Re: GET_RNG_SEED hypercall ABI? (Re: [PATCH v5 0/5] random,x86,kvm: Rework arch RNG seeds and get some from kvm)

From: Andy Lutomirski
Date: Thu Aug 28 2014 - 20:13:44 EST

On Thu, Aug 28, 2014 at 12:46 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> Il 28/08/2014 18:22, Andy Lutomirski ha scritto:
>> Is there a non-cpuid interface between QEMU and KVM for this?
> No.

Hmm. Then, assuming that someone manages to allocate a
cross-hypervisor MSR number for this, what am I supposed to do in the
KVM code? Just make it available unconditionally? I don't see why
that wouldn't work reliably, but it seems like an odd design.

>> AFAICT, even turning off cpuid bits for things like async pf doesn't
>> actually disable the MSRs (which is arguably an attack surface issue).
> No, it doesn't. You cannot disable instructions even if you hide CPUID
> bits, so KVM just extends this to MSRs (both native and paravirtual). It
> sometimes helps too, for example with a particular guest OS that does
> not necessary check CPUID for bits that are always present on Apple
> hardware...

But I bet that no one assumes that KVM paravirt MSRs are available
even if the feature bit isn't set.

Also, the one and only native feature flag I tested (rdtscp) actually
does work: RDTSCP seems to send #UD if QEMU is passed -cpu

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at