Eduardo Habkost <ehabk...@redhat.com> writes: > On Thu, Jun 03, 2021 at 01:48:29PM +0200, Vitaly Kuznetsov wrote: >> Currently, the only eVMCS version, supported by KVM (and described in TLFS) >> is '1'. When Enlightened VMCS feature is enabled, QEMU takes the supported >> eVMCS version range (from KVM_CAP_HYPERV_ENLIGHTENED_VMCS enablement) and >> puts it to guest visible CPUIDs. When (and if) eVMCS ver.2 appears a >> problem on migration is expected: it doesn't seem to be possible to migrate >> from a host supporting eVMCS ver.2 to a host, which only support eVMCS >> ver.1. > > Isn't it possible and safe to expose eVMCS ver.1 to the guest on > a host that supports ver.2?
We expose the supported range, guest is free to use any eVMCS version in the range (see below): > >> >> Hardcode eVMCS ver.1 as the result of 'hv-evmcs' enablement for now. Newer >> eVMCS versions will have to have their own enablement options (e.g. >> 'hv-evmcs=2'). >> >> Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com> >> --- >> docs/hyperv.txt | 2 +- >> target/i386/kvm/kvm.c | 16 +++++++++++----- >> 2 files changed, 12 insertions(+), 6 deletions(-) >> >> diff --git a/docs/hyperv.txt b/docs/hyperv.txt >> index a51953daa833..000638a2fd38 100644 >> --- a/docs/hyperv.txt >> +++ b/docs/hyperv.txt >> @@ -170,7 +170,7 @@ Recommended: hv-frequencies >> 3.16. hv-evmcs >> =============== >> The enlightenment is nested specific, it targets Hyper-V on KVM guests. When >> -enabled, it provides Enlightened VMCS feature to the guest. The feature >> +enabled, it provides Enlightened VMCS version 1 feature to the guest. The >> feature >> implements paravirtualized protocol between L0 (KVM) and L1 (Hyper-V) >> hypervisors making L2 exits to the hypervisor faster. The feature is >> Intel-only. >> Note: some virtualization features (e.g. Posted Interrupts) are disabled >> when >> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c >> index c676ee8b38a7..d57eede5dc81 100644 >> --- a/target/i386/kvm/kvm.c >> +++ b/target/i386/kvm/kvm.c >> @@ -1490,13 +1490,19 @@ static int hyperv_init_vcpu(X86CPU *cpu) >> ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0, >> (uintptr_t)&evmcs_version); >> >> - if (ret < 0) { >> - fprintf(stderr, "Hyper-V %s is not supported by kernel\n", >> - kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc); >> + /* >> + * KVM is required to support EVMCS ver.1. as that's what 'hv-evmcs' >> + * option sets. Note: we hardcode the maximum supported eVMCS >> version >> + * to '1' as well so 'hv-evmcs' feature is migratable even when >> (and if) >> + * ver.2 is implemented. A new option (e.g. 'hv-evmcs=2') will then >> have >> + * to be added. >> + */ >> + if (ret < 0 || (uint8_t)evmcs_version > 1) { > > Wait, do you really want to get a fatal error every time, after a > kernel upgrade? > Here, evmcs_version (returned by kvm_vcpu_enable_cap()) represents a *range* of supported eVMCS versions: (evmcs_highest_supported_version << 8) | evmcs_lowest_supported_version Currently, this is 0x101 [1..1] range. The '(uint8_t)evmcs_version > 1' check here means 'eVMCS v1' is no longer supported by KVM. This is not going to happen any time soon, but I can imagine in 10 years or so we'll be dropping v1 so the range (in theory) can be [10..2] -- which would mean eVMCS ver. 1 is NOT supported. And we can't proceed then. > I was expecting this: > > vcpu_evmcs_version = 1; /* hardcoded, but can become configurable later */ > ... > kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0, > (uintptr_t)&supported_evmcs_version); > if (ret < 0 || supported_evmcs_version < vcpu_evmcs_version) { > error_setg(...); > return; > } > cpu->hyperv_nested[0] = vcpu_evmcs_version; > > >> + error_report("Hyper-V %s verson 1 is not supported by kernel", >> + kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc); >> return ret; >> } >> - >> - cpu->hyperv_nested[0] = evmcs_version; >> + cpu->hyperv_nested[0] = (1 << 8) | 1; >> } >> >> return 0; >> -- >> 2.31.1 >> -- Vitaly