Re: [PATCH 3/5] VMX: sync VM-exit perf counters with known VM-exit reasons

[Andrew Cooper <amc96@xxxxxxxx>]

On 03/12/2021 12:05, Jan Beulich wrote:
> --- a/xen/include/asm-x86/hvm/vmx/vmx.h
> +++ b/xen/include/asm-x86/hvm/vmx/vmx.h
> @@ -220,6 +220,8 @@ static inline void pi_clear_sn(struct pi
>  #define EXIT_REASON_XSAVES              63
>  #define EXIT_REASON_XRSTORS             64
>  
> +#define EXIT_REASON_LAST                EXIT_REASON_XRSTORS
> +

Given the problems with sentinals like this in the domctl/sysctl
headers, I think this scheme would be less fragile if EXIT_REASON was an
enum.  (Also, we seriously need to reduce the scope of these headers. 
It's only just dawned on me why Intel uses EXIT_* and AMD uses VMEXIT_*)

Alternatively, what about simply this:

 #define EXIT_REASON_XSAVES              63
 #define EXIT_REASON_XRSTORS             64
+/* Remember to update VMX_PERF_EXIT_REASON_SIZE too. */

?

This avoids having yet another sentinel in the mix, and will be visible
in *every* patch review.  It also gets rid of the ifdefary in the vmexit
handler.

Another good move might be to have perfc_incra() have a printk_once()
for out-of-range indices.  That way, people using perfcounters will have
an easier time noticing if something is wrong.

~Andrew