@samitouri / QOSamiQemu / commits / 51046c654c

target/ppc/kvm: Add support for querying host compatibility mode

Add infrastructure to query the host CPU compatibility mode via the KVM_PPC_GET_COMPAT_CAPS ioctl. This allows QEMU to determine if the host is running in a compatibility mode (e.g., a Power11 processor operating in Power10 compatibility mode). The kvmppc_get_compat_caps() function issues the ioctl and returns the compat_capabilities bitmap. The kvm_ppc_host_compat_pvr() function derives the effective PVR from the bitmap using ctz64() to find the lowest set bit (highest supported compat level in IBM MSB-0 numbering). The struct kvm_ppc_compat_caps places 'size' first and userspace sets it to sizeof(struct kvm_ppc_compat_caps) before calling the ioctl. The kernel uses copy_struct_from/to_user() to handle forward and backward ABI compatibility: an older userspace with a smaller struct gets trailing fields zero-padded. When newer userspace passes a larger struct to an older kernel (usize > ksize), the kernel unconditionally returns -E2BIG and writes its own ksize back into host_compat.size. QEMU detects this, validates the returned size against KVM_PPC_COMPAT_CAPS_SIZE_VER0, and retries with that size. Additionally, cas_check_pvr() in hw/ppc/spapr_hcall.c is updated to prevent fallback to raw mode when the host is running in compatibility mode. This ensures that nested guests cannot exceed the host's compatibility level. The call is guarded with kvm_enabled() since kvm_ppc_host_compat_pvr() invokes kvm_vm_ioctl() which dereferences kvm_state; without the guard, a TCG guest on a CONFIG_KVM=y binary would segfault. If the capability is not supported or the query fails, the functions return 0, allowing fallback to existing behavior. Tested-by: Gautam Menghani <gautam@linux.ibm.com> Reviewed-by: Gautam Menghani <gautam@linux.ibm.com> Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com> Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com> Link: https://lore.kernel.org/qemu-devel/20260812170854.58709-3-amachhiw@linux.ibm.com Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>

Amit Machhiwal committed Aug 12, 2026 at 22:38 UTC 51046c654c20e0383fc6ab4f5ce52da97ad8670f

3 files changed +96

hw/ppc/spapr_hcall.c

+14

@@ -1136,6 +1136,7 @@ static uint32_t cas_check_pvr(PowerPCCPU *cpu, uint32_t max_compat,

1136 {

1137 bool explicit_match = false; /* Matched the CPU's real PVR */

1138 uint32_t best_compat = 0;

1139 + uint32_t compat_host_pvr = 0;

1140 int i;

1141

1142 /*

@@ -1163,6 +1164,19 @@ static uint32_t cas_check_pvr(PowerPCCPU *cpu, uint32_t max_compat,

1164 }

1165 }

1166

1167 + if (explicit_match && kvm_enabled()) {

1168 + compat_host_pvr = kvm_ppc_host_compat_pvr();

1169 + /*

1170 + * If the host is booted in a compatibility mode, do not try booting in

1171 + * the raw mode as it may allow KVM guests to boot with a higher CPU

1172 + * version compared to what host was booted with; which should not be

1173 + * allowed.

1174 + */

1175 + if (compat_host_pvr) {

1176 + explicit_match = false;

1177 + }

1178 + }

1179 +

1180 *raw_mode_supported = explicit_match;

1181

1182 /* Parsing finished */

target/ppc/kvm.c

+75

@@ -2602,6 +2602,81 @@ bool kvmppc_supports_ail_3(void)

2602 return cap_ail_mode_3;

2603 }

2604

2605 +#if defined(TARGET_PPC64)

2606 +static target_ulong kvmppc_get_compat_caps(void)

2607 +{

2608 + struct kvm_ppc_compat_caps host_compat;

2609 + int ret;

2610 +

2611 + if (!kvm_check_extension(kvm_state, KVM_CAP_PPC_COMPAT_CAPS)) {

2612 + return 0;

2613 + }

2614 +

2615 + /*

2616 + * Set size to sizeof(struct kvm_ppc_compat_caps) so the kernel applies

2617 + * copy_struct_from/to_user() versioning. size must be >= VER0.

2618 + */

2619 + memset(&host_compat, 0, sizeof(host_compat));

2620 + host_compat.size = sizeof(host_compat);

2621 +

2622 + ret = kvm_vm_ioctl(kvm_state, KVM_PPC_GET_COMPAT_CAPS, &host_compat);

2623 + if (ret == -E2BIG && host_compat.size >= KVM_PPC_COMPAT_CAPS_SIZE_VER0) {

2624 + /*

2625 + * Kernel is older and knows only a smaller struct version. It

2626 + * wrote back its ksize into host_compat.size. Retry with that

2627 + * size so the kernel accepts the call.

2628 + *

2629 + * When a VER1 struct is introduced, add a check here:

2630 + * if (host_compat.size >= KVM_PPC_COMPAT_CAPS_SIZE_VER1) { ... }

2631 + */

2632 + uint64_t ksize = host_compat.size;

2633 + memset(&host_compat, 0, sizeof(host_compat));

2634 + host_compat.size = ksize;

2635 + ret = kvm_vm_ioctl(kvm_state, KVM_PPC_GET_COMPAT_CAPS, &host_compat);

2636 + }

2637 +

2638 + if (ret < 0) {

2639 + error_report("KVM: failed to get host CPU compat capabilities: %s",

2640 + strerror(-ret));

2641 + return 0;

2642 + }

2643 +

2644 + return host_compat.compat_capabilities & KVM_PPC_COMPAT_BITMASK;

2645 +}

2646 +

2647 +/*

2648 + * Return the effective host PVR based on the CPU compatibility mode

2649 + * reported by KVM. Returns 0 if no compat mode is active or the

2650 + * capability is not supported, in which case the caller falls back

2651 + * to the raw hardware PVR.

2652 + */

2653 +uint32_t kvm_ppc_host_compat_pvr(void)

2654 +{

2655 + uint32_t compat_host_pvr = 0;

2656 + uint64_t cap_idx = 0;

2657 + target_ulong host_caps = kvmppc_get_compat_caps();

2658 +

2659 + if (host_caps) {

2660 + cap_idx = 1ULL << ctz64(host_caps);

2661 + switch (cap_idx) {

2662 + case KVM_PPC_COMPAT_CAP_POWER9:

2663 + compat_host_pvr = CPU_POWERPC_POWER9_DD22;

2664 + break;

2665 + case KVM_PPC_COMPAT_CAP_POWER10:

2666 + compat_host_pvr = CPU_POWERPC_POWER10_DD20;

2667 + break;

2668 + case KVM_PPC_COMPAT_CAP_POWER11:

2669 + compat_host_pvr = CPU_POWERPC_POWER11_DD20;

2670 + break;

2671 + default:

2672 + break;

2673 + }

2674 + }

2675 +

2676 + return compat_host_pvr;

2677 +}

2678 +#endif /* TARGET_PPC64 */

2679 +

2680 PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)

2681 {

2682 uint32_t host_pvr = mfpvr();

target/ppc/kvm_ppc.h

@@ -81,6 +81,8 @@ bool kvmppc_supports_ail_3(void);

81 int kvmppc_enable_hwrng(void);

82 int kvmppc_put_books_sregs(PowerPCCPU *cpu);

83 PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void);

84 +

85 +uint32_t kvm_ppc_host_compat_pvr(void);

86 void kvmppc_check_papr_resize_hpt(Error **errp);

87 int kvmppc_resize_hpt_prepare(PowerPCCPU *cpu, target_ulong flags, int shift);

88 int kvmppc_resize_hpt_commit(PowerPCCPU *cpu, target_ulong flags, int shift);

@@ -440,6 +442,11 @@ static inline PowerPCCPUClass *kvm_ppc_get_host_cpu_class(void)

442 return NULL;

443 }

444

445 +static inline uint32_t kvm_ppc_host_compat_pvr(void)

446 +{

447 + return 0;

448 +}

449 +

450 static inline void kvmppc_check_papr_resize_hpt(Error **errp)

451 {

452 }