From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> Sent: Tuesday, February 9, 2021 8:51 AM
>
> Michael Kelley <mikelley@xxxxxxxxxxxxx> 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;
>
> --
Your proposal is OK with me as well, though one downside is that it is
not compatible with current code. The return type of hv_do_hypercall()
and friends would change, so we would have to change all occurrences
in a single patch. With the helper functions, changing the code to use
them can be done incrementally.
Back when I was first working on the patches for Linux on ARM64 on
Hyper-V, I went down the path of defining a structure for the hypercall
result, but ended up abandoning it, probably because of the compatibility
issue.
But either works and is OK with me.
Michael
