Re: ** POTENTIAL FRAUD ALERT - RED HAT ** Checking Hyper-V hypercall status

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Michael Kelley <mikelley@microsoft.com> writes:

> As noted in a previous email, we don't have a consistent
> pattern for checking Hyper-V hypercall status.  Existing code and
> recent new code uses a number of variants.  The variants work, but
> a consistent pattern would improve the readability of the code, and
> be more conformant to what the Hyper-V TLFS says about hypercall
> status.  In particular, the 64 bit hypercall status contains fields that
> the TLFS says should be ignored -- evidently they are not guaranteed
> to be zero (though I've never seen otherwise).
>
> I'd propose the following helper functions to go in
> asm-generic/mshyperv.h.  The function names are relatively short
> for readability:
>
> static inline u64 hv_result(u64 status)
> {
> 	return status & HV_HYPERCALL_RESULT_MASK;
> }
>
> static inline bool hv_result_success(u64 status)
> {
> 	return hv_result(status) == HV_STATUS_SUCCESS;
> }
>
> static inline unsigned int hv_repcomp(u64 status)
> {
> 	return (status & HV_HYPERCALL_REP_COMP_MASK) >>
> 			HV_HYPERCALL_REP_COMP_OFFSET;
> }
>
> The hv_do_hypercall() function (and its 'rep' and 'fast' variants) should
> always assign the result to a u64 local variable, which is the return
> type of the functions.  Then the above functions can act on that local
> variable.  Here are some examples:
>
> 	u64		status;
> 	unsigned int	completed;
>
> 	status = hv_do_hypercall(<some args>);
> 	if (!hv_result_success(status)) {
> 		<handle error case>
> 	}
>
> 	status = hv_do_rep_hypercall(<some args>);
> 	if (hv_result(status) == HV_STATUS_INSUFFICIENT_MEMORY) {
> 		<deposit more memory pages>
> 		goto retry;
> 	} else if (!hv_result_success(status)) {
> 		<handle error case>
> 	}
> 	completed = hv_repcomp(status);
>
>
> Thoughts?

Personally, I like it and think it's going to be sufficient.

Alternatively, I can suggest we introduce something like 

struct hv_result {
	u64 status:16;
	u64 rsvd1:16;
	u64 reps_comp:12;
	u64 rsvd1:20;
};

and make hv_do_rep_hypercall() return it. So the code above will look
like:

	struct hv_result result;

	result = hv_do_rep_hypercall(<some args>);
        if (result.status) == HV_STATUS_INSUFFICIENT_MEMORY) {
                <deposit more memory pages>
                goto retry;
        } else if (result.status != HV_STATUS_SUCCESS) {
                <handle error case>
        }
        completed = result.reps_comp;

-- 
Vitaly