[PATCH v2 01/13] KVM: x86: Redefine 'longmode' as a flag for KVM_EXIT_HYPERCALL

Oliver Upton
2023-03-30T15:50:18Z

The 'longmode' field is a bit annoying as it blows an entire __u32 to
represent a boolean value. Since other architectures are looking to add
support for KVM_EXIT_HYPERCALL, now is probably a good time to clean it
up.

Redefine the field (and the remaining padding) as a set of flags.
Preserve the existing ABI by using bit 0 to indicate if the guest was in
long mode and requiring that the remaining 31 bits must be zero.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
Documentation/virt/kvm/api.rst | 3 +--
arch/x86/include/asm/kvm_host.h | 7 +++++++
arch/x86/include/uapi/asm/kvm.h | 3 +++
arch/x86/kvm/x86.c | 6 +++++-
include/uapi/linux/kvm.h | 9 +++++++--
5 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de0768d6aa..9b01e3d0e757 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6218,8 +6218,7 @@ to the byte array.
__u64 nr;
__u64 args[6];
__u64 ret;
- __u32 longmode;
- __u32 pad;
+ __u64 flags;
} hypercall;

Unused. This was once used for 'hypercall to userspace'. To implement
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 808c292ad3f4..15bda40517ff 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2204,4 +2204,11 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
KVM_X86_QUIRK_FIX_HYPERCALL_INSN | \
KVM_X86_QUIRK_MWAIT_NEVER_UD_FAULTS)

+/*
+ * KVM previously used a u32 field in kvm_run to indicate the hypercall was
+ * initiated from long mode. KVM now sets bit 0 to indicate long mode, but the
+ * remaining 31 lower bits must be 0 to preserve ABI.
+ */
+#define KVM_EXIT_HYPERCALL_MBZ GENMASK_ULL(31, 1)
+
#endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 7f467fe05d42..1a6a1f987949 100644
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -559,4 +559,7 @@ struct kvm_pmu_event_filter {
#define KVM_VCPU_TSC_CTRL 0 /* control group for the timestamp counter (TSC) */
#define KVM_VCPU_TSC_OFFSET 0 /* attribute for the TSC offset */

+/* x86-specific KVM_EXIT_HYPERCALL flags. */
+#define KVM_EXIT_HYPERCALL_LONG_MODE BIT(0)
+
#endif /* _ASM_X86_KVM_H */
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7713420abab0..27a1d5c1a018 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9803,7 +9803,11 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
vcpu->run->hypercall.args[0] = gpa;
vcpu->run->hypercall.args[1] = npages;
vcpu->run->hypercall.args[2] = attrs;
- vcpu->run->hypercall.longmode = op_64_bit;
+ vcpu->run->hypercall.flags = 0;
+ if (op_64_bit)
+ vcpu->run->hypercall.flags |= KVM_EXIT_HYPERCALL_LONG_MODE;
+
+ WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
vcpu->arch.complete_userspace_io = complete_hypercall_exit;
return 0;
}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d77aef872a0a..dd42d7dfb86c 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -341,8 +341,13 @@ struct kvm_run {
__u64 nr;
__u64 args[6];
__u64 ret;
- __u32 longmode;
- __u32 pad;
+
+ union {
+#ifndef __KERNEL__
+ __u32 longmode;
+#endif
+ __u64 flags;
+ };
} hypercall;
/* KVM_EXIT_TPR_ACCESS */
struct {
--
2.40.0.348.gf938b09366-goog