x86/hyperv: Add comments about hv_vpset and var size hypercall input args

x86/hyperv: Add comments about hv_vpset and var size hypercall input args

Current code varies in how the size of the variable size input header
for hypercalls is calculated when the input contains struct hv_vpset.
Surprisingly, this variation is correct, as different hypercalls make
different choices for what portion of struct hv_vpset is treated as part
of the variable size input header. The Hyper-V TLFS is silent on these
details, but the behavior has been confirmed with Hyper-V developers.

To avoid future confusion about these differences, add comments to
struct hv_vpset, and to hypercall call sites with input that contains
a struct hv_vpset. The comments describe the overall situation and
the calculation that should be used at each particular call site.

No functional change as only comments are updated.

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Link: https://lore.kernel.org/r/20250318214919.958953-1-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250318214919.958953-1-mhklinux@outlook.com>