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From: "Wang, Wei W" <wei.w.wang@intel.com>
To: Sean Christopherson <seanjc@google.com>
CC: "pbonzini@redhat.com" <pbonzini@redhat.com>, "kvm@vger.kernel.org"
	<kvm@vger.kernel.org>, "linux-kernel@vger.kernel.org"
	<linux-kernel@vger.kernel.org>
Subject: RE: [RFC PATCH v2 4/5] KVM: x86: Remove KVM_X86_OP_OPTIONAL
Thread-Topic: [RFC PATCH v2 4/5] KVM: x86: Remove KVM_X86_OP_OPTIONAL
Thread-Index: AQHakkz4v8XEzjLts0GEpU850B7z97FvmgoAgAAKIQA=
Date: Fri, 19 Apr 2024 15:12:27 +0000
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References: <20240419112952.15598-1-wei.w.wang@intel.com>
 <20240419112952.15598-5-wei.w.wang@intel.com> <ZiJ0mjZxlRsLwl3E@google.com>
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On Friday, April 19, 2024 9:42 PM, Sean Christopherson wrote:
> 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, respectivel=
y.
> > 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.
>=20
> 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().
>=20
> 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 (h=
opefully
> before getting to production) then to have a lurking bug just waiting to =
cause
> problems.
>=20
> > 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,
>=20
> 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.

OK, we can drop patch 4 and 5.

Btw, may I know what is the boundary between mandatory and optional hooks?
For example, when adding a new hook, what criteria should we use to determi=
ne
whether it's mandatory, thereby requiring both SVM and VMX to implement it =
(and
seems need to be merged them together?)
(I searched a bit, but didn't find it)