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target/i386: Exclude 'hv-syndbg' from 'hv-passthrough'

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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>
Link: https://lore.kernel.org/r/20240917160051.2637594-3-vkuznets@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Vitaly Kuznetsov 2024-09-17 18:00:49 +02:00 committed by Paolo Bonzini
parent bbf3810f2c
commit 7d7b9c7655
2 changed files with 14 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
docs/system/i386/hyperv.rst
View File

@ -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

7
target/i386/kvm/kvm.c
View File

@ -934,6 +934,7 @@ static struct {
uint32_t bits;
} flags[2];
uint64_t dependencies;
bool skip_passthrough;
} kvm_hyperv_properties[] = {
[HYPERV_FEAT_RELAXED] = {
.desc = "relaxed timing (hv-relaxed)",
@ -1062,7 +1063,8 @@ 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)",
@ -1471,7 +1473,8 @@ 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);
}
}