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KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode

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On IBM POWER systems, newer processor generations can operate in
compatibility modes corresponding to earlier generations. This becomes
relevant for nested virtualization, where nested KVM guests may need to
run with a specific processor compatibility level.

Currently, when running a nested KVM guest (L2) inside a Power11 pSeries
logical partition (L1) booted in Power10 compatibility mode, the guest
fails to boot while setting 'arch_compat'. This happens because the CPU
class is derived from the hardware PVR (via mfspr()), which reflects the
physical processor generation (Power11), rather than the effective
compatibility mode (Power10).

As a result, userspace may request a Power11 arch_compat for the L2
guest. However, the L1 partition, running in Power10 compatibility, has
only negotiated support up to Power10 with the Power Hypervisor (L0).
When H_GUEST_SET_STATE is invoked with a Power11 Logical PVR, the
hypervisor rejects the request, leading to a late guest boot failure:

  KVM-NESTEDv2: couldn't set guest wide elements
  [..KVM reg dump..]

This situation should be detected earlier and rejected by KVM. Without
proper validation, if userspace ignores the error, the guest may continue
to boot in Power11 raw mode on a Power10 compatibility host, which should
not be allowed.

Introduce a validation mechanism that detects unsupported arch_compat
values early in the guest initialization path. When an unsupported
arch_compat is requested (e.g., Power11 on a Power10 compatibility mode
host), kvmppc_set_arch_compat() uses cpu_has_feature(CPU_FTR_P11_PVR) to
detect the mismatch and sets arch_compat to PVR_ARCH_INVALID (0xffffffff).
This sentinel value is architecturally safe: PAPR specifies that valid
logical PVR values must have 0x0f as the first byte, ensuring 0xffffffff
lies permanently outside the specification-defined range. Setting this
value triggers kvmppc_sanity_check() to mark the vCPU as invalid by
setting vcpu->arch.sane to false. On the next vCPU run, kvmppc_vcpu_run_hv()
checks this flag and returns -EINVAL, preventing the guest from running
with an invalid processor compatibility configuration.

With this, when a Power11 arch_compat is requested on a Power10
compatibility mode host, the guest fails early during boot with:

  error: kvm run failed Invalid argument

This provides a much clearer failure mode compared to the previous
behavior where the guest could boot in Power11 raw mode (if userspace
ignored the error) or fail late during H_GUEST_SET_STATE.

Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Reviewed-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
Acked-by: Gautam Menghani <gautam@linux.ibm.com>
Cc: stable@vger.kernel.org # v6.13+
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260714175432.86388-1-amachhiw@linux.ibm.com
This commit is contained in:
Amit Machhiwal
2026-07-14 23:24:32 +05:30
committed by Madhavan Srinivasan
parent 5aabc19270
commit 884ea0283f
3 changed files with 32 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

12
arch/powerpc/include/asm/reg.h
View File

@@ -1357,6 +1357,18 @@
#define PVR_ARCH_31 0x0f000006
#define PVR_ARCH_31_P11 0x0f000007
/*
* Kernel-internal sentinel for invalid processor compatibility modes.
* PAPR specifies that the first byte of a valid logical PVR value is
* 0x0f. So 0xffffffff lies permanently outside the PAPR-defined range
* and is safe to repurpose. KVM stores it in vcpu->arch.arch_compat
* when userspace requests an unsupported compatibility mode (e.g.,
* Power11 PVR on a Power11 host booted in Power10 compat).
* kvmppc_sanity_check() detects this and prevents the vCPU from
* running with an unsupported arch_compat.
*/
#define PVR_ARCH_INVALID 0xffffffff
/* Macros for setting and retrieving special purpose registers */
#ifndef __ASSEMBLER__

15
arch/powerpc/kvm/book3s_hv.c
View File

@@ -446,7 +446,19 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
guest_pcr_bit = PCR_ARCH_300;
break;
case PVR_ARCH_31:
guest_pcr_bit = PCR_ARCH_31;
break;
case PVR_ARCH_31_P11:
/*
* Need to check this for ISA 3.1, as Power10 and
* Power11 share the same PCR. For any subsequent ISA
* versions, this will be taken care of by the guest vs
* host PCR comparison below.
*/
if (!cpu_has_feature(CPU_FTR_P11_PVR)) {
arch_compat = PVR_ARCH_INVALID;
goto out;
}
guest_pcr_bit = PCR_ARCH_31;
break;
default:
@@ -469,6 +481,7 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
return -EINVAL;
}
out:
spin_lock(&vc->lock);
vc->arch_compat = arch_compat;
kvmhv_nestedv2_mark_dirty(vcpu, KVMPPC_GSID_LOGICAL_PVR);
@@ -479,7 +492,7 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat)
vc->pcr = (host_pcr_bit - guest_pcr_bit) | PCR_MASK;
spin_unlock(&vc->lock);
return 0;
return kvmppc_sanity_check(vcpu);
}
static void kvmppc_dump_regs(struct kvm_vcpu *vcpu)

6
arch/powerpc/kvm/powerpc.c
View File

@@ -277,6 +277,12 @@ int kvmppc_sanity_check(struct kvm_vcpu *vcpu)
if (!vcpu->arch.pvr)
goto out;
#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE)
if (vcpu->arch.vcore &&
vcpu->arch.vcore->arch_compat == PVR_ARCH_INVALID)
goto out;
#endif
/* PAPR only works with book3s_64 */
if ((vcpu->arch.cpu_type != KVM_CPU_3S_64) && vcpu->arch.papr_enabled)
goto out;