linux-kernel - Re: [PATCH v2 00/25] TDX vCPU/VM creation

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Message-ID: <Z36TLcX1kOe1ltjp@google.com>
Date: Wed, 8 Jan 2025 07:01:01 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Tony Lindgren <tony.lindgren@...ux.intel.com>
Cc: Nikolay Borisov <nik.borisov@...e.com>, Rick P Edgecombe <rick.p.edgecombe@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Kai Huang <kai.huang@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, 
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	Yan Y Zhao <yan.y.zhao@...el.com>, Reinette Chatre <reinette.chatre@...el.com>
Subject: Re: [PATCH v2 00/25] TDX vCPU/VM creation

On Wed, Jan 08, 2025, Tony Lindgren wrote:
> On Tue, Jan 07, 2025 at 02:41:51PM +0200, Nikolay Borisov wrote:
> > On 7.01.25 г. 9:37 ч., Tony Lindgren wrote:
> > > --- a/arch/x86/kvm/lapic.c
> > > +++ b/arch/x86/kvm/lapic.c
> > > @@ -139,6 +139,8 @@ __read_mostly DEFINE_STATIC_KEY_FALSE(kvm_has_noapic_vcpu);
> > >   EXPORT_SYMBOL_GPL(kvm_has_noapic_vcpu);
> > >   __read_mostly DEFINE_STATIC_KEY_DEFERRED_FALSE(apic_hw_disabled, HZ);
> > > +EXPORT_SYMBOL_GPL(apic_hw_disabled);
> > 
> > Is it really required to expose this symbol? apic_hw_disabled is defined as
> > static inline in the header?

No, apic_hw_disabled can't be "static inline", because it's a variable, not a
function.

> For loadable modules yes, otherwise we'll get:
> 
> ERROR: modpost: "apic_hw_disabled" [arch/x86/kvm/kvm-intel.ko] undefined!
> 
> This is similar to the EXPORT_SYMBOL_GPL(kvm_has_noapic_vcpu) already
> there.

Heh, which is a hint that you're using the wrong helper.  TDX should check
lapic_in_kernel(), not kvm_apic_present().  The former verifies that local APIC
emulation/virtualization is handed in-kernel, i.e. by KVM.  The latter checks
that the local APIC is in-kernel *and* that the vCPU's local APIC is hardware
enabled, and checking that the local APIC is hardware enabled is unnecessary
and only works by sheer dumb luck.

The only reason kvm_create_lapic() stuffs the enable bit is to avoid toggling
the static key, which incurs costly IPIs to patch kernel text.  If
apic_hw_disabled were to be removed (which is somewhat seriously being considered),
this code would be deleted and TDX would break.

	/*
	 * Stuff the APIC ENABLE bit in lieu of temporarily incrementing
	 * apic_hw_disabled; the full RESET value is set by kvm_lapic_reset().
	 */
	vcpu->arch.apic_base = MSR_IA32_APICBASE_ENABLE;