On Tue, Dec 11, 2018 at 04:04:01PM +0100, Vitaly Kuznetsov wrote: > Roman Kagan <rkagan@xxxxxxxxxxxxx> writes: > > > On Tue, Dec 11, 2018 at 02:28:14PM +0100, Vitaly Kuznetsov wrote: > >> Roman Kagan <rkagan@xxxxxxxxxxxxx> 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. > > > > Yes, I'm not sure if we have to have full consistency between > KVM_GET_SUPPORTED_CPUID and KVM_GET_SUPPORTED_HV_CPUID. Neither do I. I just noticed the difference and thought it might matter. > >> 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. > > Not necessarily. If very dumb userspace does 'host passthrough' for > Hyper-V features without doing anything (e.g. not enabling Enlightened > VMCS) it will just put the result of KVM_GET_SUPPORTED_HV_CPUID in guest > facing CPUIDs and it will all work. In case eVMCS was previously enabled > it again just copies everything and this still works. > > We don't probably need this for Qemu though. If you think it would be > better to have HYPERV_CPUID_NESTED_FEATURES returned regardless of eVMCS > enablement I'm ready to budge) I have no opinion on this. I hope Paolo, who requested the feature, can explain the desired semantics :) Roman.
