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arch: x86: Add comprehensive Hyper-V CPUID features for nested virtualization #1

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arch: x86: Add comprehensive Hyper-V CPUID features for nested virtualization #1

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135 changes: 124 additions & 11 deletions arch/src/x86_64/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -724,14 +724,44 @@ pub fn generate_common_cpuid(
}

if config.kvm_hyperv {
// Remove conflicting entries
// Detect if the host supports nested virtualization.
// On AMD: Check CPUID 0x8000_0001.ECX bit 2 (SVM).
// On Intel: Check CPUID 0x1.ECX bit 5 (VMX).
let nested_virt_supported = {
let vendor = hypervisor.get_cpu_vendor();
let mut has_nested = false;

for entry in cpuid.iter() {
if matches!(vendor, CpuVendor::AMD) && entry.function == 0x8000_0001 {
has_nested = (entry.ecx & (1 << 2)) != 0;
if has_nested {
info!(
"AMD SVM nested virtualization detected and will be exposed to guest"
);
}
break;
} else if matches!(vendor, CpuVendor::Intel) && entry.function == 0x1 {
has_nested = (entry.ecx & (1 << 5)) != 0;
if has_nested {
info!(
"Intel VMX nested virtualization detected and will be exposed to guest"
);
}
break;
}
}
has_nested
};

// Remove conflicting entries.
cpuid.retain(|c| c.function != 0x4000_0000);
cpuid.retain(|c| c.function != 0x4000_0001);
// See "Hypervisor Top Level Functional Specification" for details
// Compliance with "Hv#1" requires leaves up to 0x4000_000a

// See "Hypervisor Top Level Functional Specification" (TLFS) for details.
// Compliance with "Hv#1" requires leaves up to 0x4000_000a.
cpuid.push(CpuIdEntry {
function: 0x40000000,
eax: 0x4000000a, // Maximum cpuid leaf
eax: 0x4000000a, // Maximum cpuid leaf.
ebx: 0x756e694c, // "Linu"
ecx: 0x564b2078, // "x KV"
edx: 0x7648204d, // "M Hv"
Expand All @@ -748,26 +778,109 @@ pub fn generate_common_cpuid(
ebx: 0xa0000, // "Version"
..Default::default()
});

// Leaf 0x40000003: Partition privilege flags (EAX) and implementation features (EDX).
// These control what the guest hypervisor is allowed to do.
// Base features always exposed (for basic Windows operation):
let mut features_eax = (1 << 1) // HV_TIME_REF_COUNT_AVAILABLE
| (1 << 2) // HV_SYNIC_AVAILABLE
| (1 << 3) // HV_SYNTIMERS_AVAILABLE
| (1 << 9); // HV_REFERENCE_TSC_AVAILABLE

let mut features_ebx = 0u32;

let mut features_edx = 1 << 3; // HV_CPU_DYNAMIC_PARTITIONING_AVAILABLE

// If nested virtualization is supported, expose additional features required
// for Windows to operate as an L1 hypervisor (for nested Hyper-V VMs and WSL2).
// Based on QEMU's implementation (target/i386/kvm/hyperv-proto.h).
if nested_virt_supported {
info!("Enabling Hyper-V nested enlightenments for Windows L1 hypervisor support");

// Additional partition privileges needed for nested operation.
features_eax |= (1 << 0) // HV_VP_RUNTIME_AVAILABLE
| (1 << 4) // HV_APIC_ACCESS_AVAILABLE
| (1 << 5) // HV_HYPERCALL_AVAILABLE - Critical for nested hypercalls
| (1 << 6) // HV_VP_INDEX_AVAILABLE
| (1 << 7) // HV_RESET_AVAILABLE
| (1 << 11) // HV_ACCESS_FREQUENCY_MSRS - Critical for TSC/APIC frequency
| (1 << 13); // HV_ACCESS_REENLIGHTENMENTS_CONTROL

// SynIC event/message posting (used by nested VMs).
features_ebx |= (1 << 4) // HV_POST_MESSAGES
| (1 << 5); // HV_SIGNAL_EVENTS

// Additional implementation features for nested scenarios.
features_edx |= (1 << 0) // HV_MWAIT_AVAILABLE
| (1 << 5) // HV_GUEST_IDLE_STATE_AVAILABLE
| (1 << 8) // HV_FREQUENCY_MSRS_AVAILABLE - Pairs with bit 11 in EAX
| (1 << 10) // HV_GUEST_CRASH_MSR_AVAILABLE
| (1 << 19); // HV_STIMER_DIRECT_MODE_AVAILABLE
}

info!(
"Hyper-V CPUID 0x40000003: EAX={features_eax:#010x} EBX={features_ebx:#010x} EDX={features_edx:#010x} (nested={nested_virt_supported})"
);

cpuid.push(CpuIdEntry {
function: 0x4000_0003,
eax: (1 << 1) // AccessPartitionReferenceCounter
| (1 << 2) // AccessSynicRegs
| (1 << 3) // AccessSyntheticTimerRegs
| (1 << 9), // AccessPartitionReferenceTsc
edx: 1 << 3, // CPU dynamic partitioning
eax: features_eax,
ebx: features_ebx,
edx: features_edx,
..Default::default()
});

// Leaf 0x40000004: Hypervisor recommendations.
let mut recommendations_eax = 1 << 5; // HV_RELAXED_TIMING_RECOMMENDED

if nested_virt_supported {
// Recommend hypercall-based operations for better nested performance.
recommendations_eax |= (1 << 3) // HV_APIC_ACCESS_RECOMMENDED
| (1 << 4) // HV_SYSTEM_RESET_RECOMMENDED
| (1 << 10) // HV_CLUSTER_IPI_RECOMMENDED
| (1 << 11); // HV_EX_PROCESSOR_MASKS_RECOMMENDED
}

cpuid.push(CpuIdEntry {
function: 0x4000_0004,
eax: 1 << 5, // Recommend relaxed timing
eax: recommendations_eax,
..Default::default()
});

// Leaf 0x40000005: Implementation limits.
cpuid.push(CpuIdEntry {
function: 0x4000_0005,
..Default::default()
});
for i in 0x4000_0005..=0x4000_000a {

// Leaves 0x40000006-0x40000009: Reserved/empty.
for i in 0x4000_0006..=0x4000_0009 {
cpuid.push(CpuIdEntry {
function: i,
..Default::default()
});
}

// Leaf 0x4000000A: Nested hypervisor features.
// Only exposed if nested virtualization is supported.
if nested_virt_supported {
let nested_features_eax = (1 << 17) // HV_NESTED_DIRECT_FLUSH - Direct TLB flush for nested VMs
| (1 << 19); // HV_NESTED_MSR_BITMAP - MSR bitmap enlightenment

info!("Hyper-V CPUID 0x4000000A: EAX={nested_features_eax:#010x} (nested features)");

cpuid.push(CpuIdEntry {
function: 0x4000_000a,
eax: nested_features_eax,
..Default::default()
});
} else {
// Still create the leaf but with all zeros to maintain leaf structure.
cpuid.push(CpuIdEntry {
function: 0x4000_000a,
..Default::default()
});
}
}

Ok(cpuid)
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