Re: [PATCH 01/22] KVM: x86: Drop unnecessary and confusing KVM_X86_OP_NULL macro

[Paolo Bonzini <pbonzini@redhat.com>]

On 1/28/22 16:42, Sean Christopherson wrote:
> On Fri, Jan 28, 2022, Paolo Bonzini wrote:
>> On 1/28/22 01:51, Sean Christopherson wrote:
>>> Drop KVM_X86_OP_NULL, which is superfluous and confusing.  The macro is
>>> just a "pass-through" to KVM_X86_OP; it was added with the intent of
>>> actually using it in the future, but that obviously never happened.  The
>>> name is confusing because its intended use was to provide a way for
>>> vendor implementations to specify a NULL pointer, and even if it were
>>> used, wouldn't necessarily be synonymous with declaring a kvm_x86_op as
>>> DEFINE_STATIC_CALL_NULL.
>>>
>>> Lastly, actually using KVM_X86_OP_NULL as intended isn't a maintanable
>>> approach, e.g. bleeds vendor details into common x86 code, and would
>>> either be prone to bit rot or would require modifying common x86 code
>>> when modifying a vendor implementation.
>>
>> I have some patches that redefine KVM_X86_OP_NULL as "must be used with
>> static_call_cond".  That's a more interesting definition, as it can be used
>> to WARN if KVM_X86_OP is used with a NULL function pointer.
> 
> I'm skeptical that will actually work well and be maintainble.  E.g. sync_pir_to_ir()
> must be explicitly check for NULL in apic_has_interrupt_for_ppr(), forcing that path
> to do static_call_cond() will be odd.  Ditto for ops that are wired up to ioctl()s,
> e.g. the confidential VM stuff, and for ops that are guarded by other stuff, e.g. the
> hypervisor timer.
> 
> Actually, it won't just be odd, it will be impossible to disallow NULL a pointer
> for KVM_X86_OP and require static_call_cond() for KVM_X86_OP_NULL.  static_call_cond()
> forces the return to "void", so any path that returns a value needs to be manually
> guarded and can't use static_call_cond(), e.g.

You're right and I should have looked up the series instead of going by 
memory.  What I did was mostly WARNing on KVM_X86_OP that sets NULL, as 
non-NULL ops are the common case.  I also added KVM_X86_OP_RET0 to 
remove some checks on kvm_x86_ops for ops that return a value.

All in all I totally agree with patches 2-11 and will apply them (patch 
2 to 5.17 even, as a prerequisite to fix the AVIC race).  Several of 
patches 13-21 are also mostly useful as it clarifies the code, and the 
others I guess are okay in the context of a coherent series though 
probably they would have been rejected as one-offs.  However, patches 12 
and 22 are unnecessary uses of the C preprocessor in my opinion.

Paolo