linux-kernel - Re: [PATCH 2/4] KVM: x86: Register emergency virt callback in common code, via kvm_x86

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Date: Mon, 13 May 2024 09:17:45 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] KVM: x86: Register emergency virt callback in common
 code, via kvm_x86_ops

On Mon, May 13, 2024, Kai Huang wrote:
> On Fri, 2024-04-26 at 10:08 -0700, Sean Christopherson wrote:
> > On Fri, Apr 26, 2024, Chao Gao wrote:
> > > > diff --git a/arch/x86/kvm/vmx/x86_ops.h b/arch/x86/kvm/vmx/x86_ops.h
> > > > index 502704596c83..afddfe3747dd 100644
> > > > --- a/arch/x86/kvm/vmx/x86_ops.h
> > > > +++ b/arch/x86/kvm/vmx/x86_ops.h
> > > > @@ -15,6 +15,7 @@ void vmx_hardware_unsetup(void);
> > > > int vmx_check_processor_compat(void);
> > > > int vmx_hardware_enable(void);
> > > > void vmx_hardware_disable(void);
> > > > +void vmx_emergency_disable(void);
> > > > int vmx_vm_init(struct kvm *kvm);
> > > > void vmx_vm_destroy(struct kvm *kvm);
> > > > int vmx_vcpu_precreate(struct kvm *kvm);
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index e9ef1fa4b90b..12e88aa2cca2 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -9797,6 +9797,8 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> > > > 
> > > > 	kvm_ops_update(ops);
> > > > 
> > > > +	cpu_emergency_register_virt_callback(kvm_x86_ops.emergency_disable);
> > > > +
> > > 
> > > vmx_emergency_disable() accesses loaded_vmcss_on_cpu but now it may be called
> > > before loaded_vmcss_on_cpu is initialized. This may be not a problem for now
> > > given the check for X86_CR4_VMXE  in vmx_emergency_disable(). But relying on
> > > that check is fragile. I think it is better to apply the patch below from Isaku
> > > before this patch.
> > > 
> > > https://lore.kernel.org/kvm/c1b7f0e5c2476f9f565acda5c1e746b8d181499b.1708933498.git.isaku.yamahata@intel.com/
> > 
> > Agreed, good eyeballs, and thanks for the reviews!
> > 
> 
> I think we can even move registering this emergency disable to
> hardware_enable_all()?  It seems there's no reason to register the
> callback if hardware_enable_all() hasn't been attempted.

Hmm, we could.  I don't know that it'd be worth doing though.  I suppose one
could argue that it would allow out-of-tree hypervisors to more easily co-exist
with KVM, but I haven't heard/seen anyone crying for that.  And it would be nice
to have all of this code in one location.  

I think we'd need more explicit synchronization if the callback is registered
on-demand, but that should be a relatively minor, if it's even needed.

So yeah, I'll give this a shot and go this route for v2 if it works out.