Re: [PATCH v2 4/7] x86/kvm/hyper-v: Introduce KVM_GET_SUPPORTED_HV_CPUID

[Roman Kagan <rkagan@virtuozzo.com>]

On Tue, Dec 11, 2018 at 02:28:14PM +0100, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@virtuozzo.com> writes:
> 
> > On Mon, Dec 10, 2018 at 06:21:56PM +0100, Vitaly Kuznetsov wrote:
> 
> >> +
> >> +Currently, the following list of CPUID leaves are returned:
> >> + HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS
> >> + HYPERV_CPUID_INTERFACE
> >> + HYPERV_CPUID_VERSION
> >> + HYPERV_CPUID_FEATURES
> >> + HYPERV_CPUID_ENLIGHTMENT_INFO
> >> + HYPERV_CPUID_IMPLEMENT_LIMITS
> >> + HYPERV_CPUID_NESTED_FEATURES
> >> +
> >> +HYPERV_CPUID_NESTED_FEATURES leaf is only exposed when Enlightened VMCS was
> >> +enabled on the corresponding vCPU (KVM_CAP_HYPERV_ENLIGHTENED_VMCS).
> >
> > IOW the output of ioctl(KVM_GET_SUPPORTED_HV_CPUID) depends on
> > whether ioctl(KVM_ENABLE_CAP, KVM_CAP_HYPERV_ENLIGHTENED_VMCS) has
> > already been called on that vcpu?  I wonder if this fits the intended
> > usage?
> 
> I added HYPERV_CPUID_NESTED_FEATURES in the list (and made the new ioctl
> per-cpu and not per-vm) for consistency. *In theory*
> KVM_CAP_HYPERV_ENLIGHTENED_VMCS is also enabled per-vcpu so some
> hypothetical userspace can later check enabled eVMCS versions (which can
> differ across vCPUs!) with KVM_GET_SUPPORTED_HV_CPUID. We will also have
> direct tlb flush and other nested features there so to avoid addning new
> KVM_CAP_* for them we need the CPUID.

This is different from how KVM_GET_SUPPORTED_CPUID is used: QEMU assumes
that its output doesn't change between calls, and even caches the result
calling the ioctl only once.

> Another thing I'm thinking about is something like 'hv_all' cpu flag for
> Qemu which would enable everything by setting guest CPUIDs to what
> KVM_GET_SUPPORTED_HV_CPUID returns. In that case it would also be
> convenient to have HYPERV_CPUID_NESTED_FEATURES properly filled (or not
> filled when eVMCS was not enabled).

I think this is orthogonal to the way you obtain capability info from
the kernel.

Roman.