Re: [RFC PATCH v2 4/5] KVM: x86: Remove KVM_X86_OP_OPTIONAL

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 19, 2024, Wei Wang wrote:
> KVM_X86_OP and KVM_X86_OP_OPTIONAL were utilized to define and execute
> static_call_update() calls on mandatory and optional hooks, respectively.
> Mandatory hooks were invoked via static_call() and necessitated definition
> due to the presumption that an undefined hook (i.e., NULL) would cause
> static_call() to fail. This assumption no longer holds true as
> static_call() has been updated to treat a "NULL" hook as a NOP on x86.
> Consequently, the so-called mandatory hooks are no longer required to be
> defined, rendering them non-mandatory. 

This is wrong.  They absolutely are mandatory.  The fact that static_call() doesn't
blow up doesn't make them optional.  If a vendor neglects to implement a mandatory
hook, KVM *will* break, just not immediately on the static_call().

The static_call() behavior is actually unfortunate, as KVM at least would prefer
that it does explode on a NULL point.  I.e. better to crash the kernel (hopefully
before getting to production) then to have a lurking bug just waiting to cause
problems.

> This eliminates the need to differentiate between mandatory and optional
> hooks, allowing a single KVM_X86_OP to suffice.
> 
> So KVM_X86_OP_OPTIONAL and the WARN_ON() associated with KVM_X86_OP are
> removed to simplify usage, 

Just in case it isn't clear, I am very strongly opposed to removing KVM_X86_OP_OPTIONAL()
and the WARN_ON() protection to ensure mandatory ops are implemented.