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

[Andy Lutomirski]

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
host,-rdtscp.

--Andy
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