Re: [PATCH v4 1/2] KVM: MMU: Introduce 'INVALID_GFN' and use it for GFN values

[Sean Christopherson <seanjc@xxxxxxxxxx>]

On Fri, Dec 16, 2022, Yu Zhang wrote:
> Currently, KVM xen and its shared info selftest code uses
> 'GPA_INVALID' for GFN values, but actually it is more accurate
> to use the name 'INVALID_GFN'. So just add a new definition
> and use it.
> 
> No functional changes intended.
> 
> Suggested-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kvm/xen.c                                   | 4 ++--
>  include/linux/kvm_types.h                            | 1 +
>  tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c | 4 ++--
>  3 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index d7af40240248..6908a74ab303 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -41,7 +41,7 @@ static int kvm_xen_shared_info_init(struct kvm *kvm, gfn_t gfn)
>  	int ret = 0;
>  	int idx = srcu_read_lock(&kvm->srcu);
>  
> -	if (gfn == GPA_INVALID) {
> +	if (gfn == INVALID_GFN) {

Grrr!  This magic value is ABI, as "gfn == -1" yields different behavior than a
random, garbage gfn.
                                                                                
So, sadly, we can't simply introduce INVALID_GFN here, and instead need to do
something like:

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 20522d4ba1e0..2d31caaf812c 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1766,6 +1766,7 @@ struct kvm_xen_hvm_attr {
                __u8 vector;
                __u8 runstate_update_flag;
                struct {
+#define KVM_XEN_INVALID_GFN    (~0ull)
                        __u64 gfn;
                } shared_info;
                struct {

>  		kvm_gpc_deactivate(gpc);
>  		goto out;
>  	}