| Message ID | 20210608120817.1325125-4-vkuznets@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | i386: KVM: expand Hyper-V features early | expand |
On Tue, Jun 08, 2021 at 02:08:11PM +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. Should we rewrite this as "it wouldn't be possible to migrate", as this patch fixes the problem and makes it possible? > > 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 <vkuznets@redhat.com> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Eduardo Habkost <ehabkost@redhat.com> writes: > On Tue, Jun 08, 2021 at 02:08:11PM +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. > > Should we rewrite this as "it wouldn't be possible to migrate", > as this patch fixes the problem and makes it possible? Yes, no problem with such amendment. Currently, there's no issue as EVMCSv2 just doesn't exist. We, however, expect it to appear some time in the future and this change allows us to re-use KVM_CAP_HYPERV_ENLIGHTENED_VMCS in KVM without (potentially) breaking migrations. Note: the migration will only be broken when we migrate to KVM/QEMU which does not support EVMCSv2 *and* when the guest is already using it. As we expose the range of supported versions, it is possible that guests (esp. older Hyper-V versions) will stick to 'v1' even when 'v2' is supported. > >> >> 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 <vkuznets@redhat.com> > > Reviewed-by: Eduardo Habkost <ehabkost@redhat.com> Thanks! Please let me know if expect v9 with amended commit message or if you're able to alter it upon commit.
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..13d63f576b88 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -1406,6 +1406,21 @@ static int hyperv_fill_cpuids(CPUState *cs, static Error *hv_passthrough_mig_blocker; static Error *hv_no_nonarch_cs_mig_blocker; +/* Checks that the exposed eVMCS version range is supported by KVM */ +static bool evmcs_version_supported(uint16_t evmcs_version, + uint16_t supported_evmcs_version) +{ + uint8_t min_version = evmcs_version & 0xff; + uint8_t max_version = evmcs_version >> 8; + uint8_t min_supported_version = supported_evmcs_version & 0xff; + uint8_t max_supported_version = supported_evmcs_version >> 8; + + return (min_version >= min_supported_version) && + (max_version <= max_supported_version); +} + +#define DEFAULT_EVMCS_VERSION ((1 << 8) | 1) + static int hyperv_init_vcpu(X86CPU *cpu) { CPUState *cs = CPU(cpu); @@ -1485,17 +1500,33 @@ static int hyperv_init_vcpu(X86CPU *cpu) } if (hyperv_feat_enabled(cpu, HYPERV_FEAT_EVMCS)) { - uint16_t evmcs_version; + uint16_t evmcs_version = DEFAULT_EVMCS_VERSION; + uint16_t supported_evmcs_version; ret = kvm_vcpu_enable_cap(cs, KVM_CAP_HYPERV_ENLIGHTENED_VMCS, 0, - (uintptr_t)&evmcs_version); + (uintptr_t)&supported_evmcs_version); + /* + * 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) { - fprintf(stderr, "Hyper-V %s is not supported by kernel\n", - kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc); + error_report("Hyper-V %s is not supported by kernel", + kvm_hyperv_properties[HYPERV_FEAT_EVMCS].desc); return ret; } + if (!evmcs_version_supported(evmcs_version, supported_evmcs_version)) { + error_report("eVMCS version range [%d..%d] is not supported by " + "kernel (supported: [%d..%d])", evmcs_version & 0xff, + evmcs_version >> 8, supported_evmcs_version & 0xff, + supported_evmcs_version >> 8); + return -ENOTSUP; + } + cpu->hyperv_nested[0] = evmcs_version; }
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. 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 <vkuznets@redhat.com> --- docs/hyperv.txt | 2 +- target/i386/kvm/kvm.c | 39 +++++++++++++++++++++++++++++++++++---- 2 files changed, 36 insertions(+), 5 deletions(-)