Re: [PATCH] KVM: x86: hyper-v: Fix 'using uninitialized value' Coverity warning

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 02, 2022, Vitaly Kuznetsov wrote:
> In kvm_hv_flush_tlb(), 'data_offset' and 'consumed_xmm_halves' variables
> are used in a mutually exclusive way: in 'hc->fast' we count in 'XMM
> halves' and increase 'data_offset' otherwise. Coverity discovered, that in
> one case both variables are incremented unconditionally. This doesn't seem
> to cause any issues as the only user of 'data_offset'/'consumed_xmm_halves'
> data is kvm_hv_get_tlb_flush_entries() ->  kvm_hv_get_hc_data() which also
> takes into account 'hc->fast' but is still worth fixing.

If those calls aren't inlined, then 32-bit Hyper-V will be "consuming" uninitialized
data when pushing parameters onto the stack.  It won't cause real problems, but
checkers might complain.

What about shoving this metadata into "struct kvm_hv_hcall" as a union?  That'd
help convey that the two are mutually exclusive, would provide a place to document
said exclusion, and would yield a nice cleanup too by eliminating multiple params
from various functions.