Windows with Hyper-V role enabled doesn't boot with 'hv-passthrough' when no debugger is configured, this significantly limits the usefulness of the feature as there's no support for subtracting Hyper-V features from CPU flags at this moment (e.g. "-cpu host,hv-passthrough,-hv-syndbg" does not work). While this is also theoretically fixable, 'hv-syndbg' is likely very special and unneeded in the default set. Genuine Hyper-V doesn't seem to enable it either.
Introduce 'skip_passthrough' flag to 'kvm_hyperv_properties' and use it as one-off to skip 'hv-syndbg' when enabling features in 'hv-passthrough' mode. Note, "-cpu host,hv-passthrough,hv-syndbg" can still be used if needed. As both 'hv-passthrough' and 'hv-syndbg' are debug features, the change should not have any effect on production environments. Signed-off-by: Vitaly Kuznetsov <vkuzn...@redhat.com> --- docs/system/i386/hyperv.rst | 13 +++++++++---- target/i386/kvm/kvm.c | 7 +++++-- 2 files changed, 14 insertions(+), 6 deletions(-) diff --git a/docs/system/i386/hyperv.rst b/docs/system/i386/hyperv.rst index 2505dc4c86e0..009947e39141 100644 --- a/docs/system/i386/hyperv.rst +++ b/docs/system/i386/hyperv.rst @@ -262,14 +262,19 @@ Supplementary features ``hv-passthrough`` In some cases (e.g. during development) it may make sense to use QEMU in 'pass-through' mode and give Windows guests all enlightenments currently - supported by KVM. This pass-through mode is enabled by "hv-passthrough" CPU - flag. + supported by KVM. Note: ``hv-passthrough`` flag only enables enlightenments which are known to QEMU (have corresponding 'hv-' flag) and copies ``hv-spinlocks`` and ``hv-vendor-id`` values from KVM to QEMU. ``hv-passthrough`` overrides all other 'hv-' settings on - the command line. Also, enabling this flag effectively prevents migration as the - list of enabled enlightenments may differ between target and destination hosts. + the command line. + + Note: ``hv-passthrough`` does not enable ``hv-syndbg`` which can prevent certain + Windows guests from booting when used without proper configuration. If needed, + ``hv-syndbg`` can be enabled additionally. + + Note: ``hv-passthrough`` effectively prevents migration as the list of enabled + enlightenments may differ between target and destination hosts. ``hv-enforce-cpuid`` By default, KVM allows the guest to use all currently supported Hyper-V diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c index 4009fcfd6b29..cf0449968f66 100644 --- a/target/i386/kvm/kvm.c +++ b/target/i386/kvm/kvm.c @@ -913,6 +913,7 @@ static struct { uint32_t bits; } flags[2]; uint64_t dependencies; + bool skip_passthrough; } kvm_hyperv_properties[] = { [HYPERV_FEAT_RELAXED] = { .desc = "relaxed timing (hv-relaxed)", @@ -1041,7 +1042,8 @@ static struct { {.func = HV_CPUID_FEATURES, .reg = R_EDX, .bits = HV_FEATURE_DEBUG_MSRS_AVAILABLE} }, - .dependencies = BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_RELAXED) + .dependencies = BIT(HYPERV_FEAT_SYNIC) | BIT(HYPERV_FEAT_RELAXED), + .skip_passthrough = true, }, [HYPERV_FEAT_MSR_BITMAP] = { .desc = "enlightened MSR-Bitmap (hv-emsr-bitmap)", @@ -1450,7 +1452,8 @@ bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp) * hv_build_cpuid_leaf() uses this info to build guest CPUIDs. */ for (feat = 0; feat < ARRAY_SIZE(kvm_hyperv_properties); feat++) { - if (hyperv_feature_supported(cs, feat)) { + if (hyperv_feature_supported(cs, feat) && + !kvm_hyperv_properties[feat].skip_passthrough) { cpu->hyperv_features |= BIT(feat); } } -- 2.46.0