@@ -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
@@ -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);
}
}
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 <vkuznets@redhat.com> --- docs/system/i386/hyperv.rst | 13 +++++++++---- target/i386/kvm/kvm.c | 7 +++++-- 2 files changed, 14 insertions(+), 6 deletions(-)