Eduardo Habkost <ehabk...@redhat.com> writes: > On Fri, Jun 04, 2021 at 09:28:15AM +0200, Vitaly Kuznetsov wrote: >> 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): > > Oh, I didn't notice the returned value was a range. > >> >> > >> >> >> >> 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. > > Where is this documented? The only reference to > KVM_CAP_HYPERV_ENLIGHTENED_VMCS I've found in linux/Documentation is this > single sentence: > > | - HYPERV_CPUID_NESTED_FEATURES leaf and HV_X64_ENLIGHTENED_VMCS_RECOMMENDED > | feature bit are only exposed when Enlightened VMCS was previously enabled > | on the corresponding vCPU (KVM_CAP_HYPERV_ENLIGHTENED_VMCS). >
It seems that KVM_CAP_HYPERV_ENLIGHTENED_VMCS's documentation is indeed missing in KVM. > >> >> 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) { > > Can we start with something that won't need to be rewritten after we change > the > guest evmcs version range? e.g.: > > static bool evmcs_version_supported(uint16_t version, uint32_t supported) > { > uint8_t min_ver = version; > uint8_t max_ver = version >> 8; > uint8_t min_supported = supported; > uint8_t max_supported = supported >> 8; > return (min_ver >= min_supported) && (max_ver <= max_supported); > } > ... > #define DEFAULT_EVMCS_VERSION ((1 << 8) | 1) > ... > uint16_t evmcs_version = DEFAULT_EVMCS_VERSION; > int ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0, > (uintptr_t)&supported_evmcs_version); > if (ret < 0) { > fprintf(...) > return ret; > } > if (!evmcs_version_supported(evmcs_version, supported_evmcs_version)) { > fprintf(...) > return -ENOTSUP; > } > cpu->hyperv_nested[0] = evmcs_version; > Sure, I don't have anything against an attempt to make this future proof. Will use this in v8, thanks! -- Vitaly