KVM: x86: Redefine 'longmode' as a flag for KVM_EXIT_HYPERCALL

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>
Acked-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-2-oliver.upton@linux.dev

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

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 */

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 */

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;
 	}

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 {