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

[Oliver Upton <oliver.upton@linux.dev>]

On Mon, Feb 13, 2023 at 04:01:29PM +0000, Sean Christopherson wrote:
> 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.

And surrender the opportunity to write a smartass commit message? I
think not.


> 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;
> +                       };

You could pull further shenanigans with the x86 residue by guarding
longmode with #ifndef __KERNEL__ and using a macro to raise the
'longmode' flag.

Then we can all pretend it never existed :)

-- 
Thanks,
Oliver