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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2022-01-31 at 15:56 +0100, Paolo Bonzini wrote:
> 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.
> 

I will send my patches very very soon - I'll rebase on top of this,
and review this patch series soon as well.

Best regards,
	Maxim Levitsky

> Paolo
>