This adds a new powerpc-specific KVM_CAP_SPAPR_RESIZE_HPT capability to
advertise whether KVM is capable of handling the PAPR extensions for
resizing the hashed page table during guest runtime.

At present, HPT resizing is possible with KVM PR without kernel
modification, since the HPT is managed within qemu.  It's not possible yet
with KVM HV, because the HPT is managed by KVM.  At present, qemu has to
use other capabilities which (by accident) reveal whether PR or HV is in
use to know if it can advertise HPT resizing capability to the guest.

To avoid ambiguity with existing kernels, the encoding is a bit odd.
    0 means "unknown" since that's what previous kernels will return
    1 means "HPT resize possible if available if and only if the HPT is allocated in
      userspace, rather than in the kernel".  Userspace can check
      KVM_CAP_PPC_ALLOC_HTAB to determine if that's the case.  In practice
      this will give the same results as userspace's fallback check.
    2 will mean "HPT resize available and implemented via ioctl()s
      KVM_PPC_RESIZE_HPT_PREPARE and KVM_PPC_RESIZE_HPT_COMMIT"

For now we always return 1, but the intention is to return 2 once HPT
resize is implemented for KVM HV.

Signed-off-by: David Gibson <redacted>
---
 arch/powerpc/kvm/powerpc.c |  3 +++
 include/uapi/linux/kvm.h   | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index efd1183..bb23923 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c

@@ -605,6 +605,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_SPAPR_MULTITCE:
 		r = 1;
 		break;
+	case KVM_CAP_SPAPR_RESIZE_HPT:
+		r = 1; /* resize allowed only if HPT is outside kernel */
+		break;
 #endif
 	case KVM_CAP_PPC_HTM:
 		r = cpu_has_feature(CPU_FTR_TM_COMP) &&

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index cac48ed..904afe0 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h

@@ -685,6 +685,12 @@ struct kvm_ppc_smmu_info {
 	struct kvm_ppc_one_seg_page_size sps[KVM_PPC_PAGE_SIZES_MAX_SZ];
 };
 
+/* for KVM_PPC_RESIZE_HPT_{PREPARE,COMMIT} */
+struct kvm_ppc_resize_hpt {
+	__u64 flags;
+	__u32 shift;
+};