[RFC PATCH v2 5/6] KVM: arm64: Add support for KVM_EXIT_HYPERCALL

[Sean Christopherson]

On Sat, Feb 11, 2023, Oliver Upton wrote:
> Finally, we need a flag to let userspace know what conduit instruction
> was used (i.e. SMC vs. HVC). Redefine the remaining padding in
> kvm_run::hypercall to accomplish this. Let's all take a moment
> to admire the flowers and see how 'longmode' tied up a full u32 in the
> UAPI. Weep.

I don't think it has to, at least not for other architectures.  No other arch
currently supports KVM_EXIT_HYPERCALL, so breakage is extremely unlikely.  E.g.
we can likely get away with this, and then just have x86 add new flags at bit 32.

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 55155e262646..7d3ad820d55c 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -341,8 +341,14 @@ struct kvm_run {
                        __u64 nr;
                        __u64 args[6];
                        __u64 ret;
-                       __u32 longmode;
-                       __u32 pad;
+                       union {
+                               /*
+                                * Long Mode, a.k.a. 64-bit mode, takes up the
+                                * first 32 flags on x86 (historical sludge).
+                                */
+                               __u32 longmode;
+                               __u64 flags;
+                       };
                } hypercall;
                /* KVM_EXIT_TPR_ACCESS */
                struct {