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Date: Wed, 10 Apr 2024 08:29:42 -0700
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References: <cover.1708933498.git.isaku.yamahata@intel.com>
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Message-ID: <ZhawUG0BduPVvVhN@google.com>
Subject: Re: [PATCH v19 023/130] KVM: TDX: Initialize the TDX module when
 loading the KVM intel kernel module
From: Sean Christopherson <seanjc@google.com>
To: Kai Huang <kai.huang@intel.com>
Cc: Isaku Yamahata <isaku.yamahata@intel.com>, Tina Zhang <tina.zhang@intel.com>, 
	Hang Yuan <hang.yuan@intel.com>, Bo Chen <chen.bo@intel.com>, 
	"sagis@google.com" <sagis@google.com>, "isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Erdem Aktas <erdemaktas@google.com>, 
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>, "pbonzini@redhat.com" <pbonzini@redhat.com>, 
	"isaku.yamahata@linux.intel.com" <isaku.yamahata@linux.intel.com>
Content-Type: text/plain; charset="us-ascii"

On Wed, Apr 10, 2024, Kai Huang wrote:
> On Fri, 2024-03-22 at 14:23 -0700, Isaku Yamahata wrote:
> > > > +	r = atomic_read(&enable.err);
> > > > +	if (!r)
> > > > +		r = tdx_module_setup();
> > > > +	else
> > > > +		r = -EIO;
> > > > +	on_each_cpu(vmx_off, &enable.enabled, true);
> > > > +	cpus_read_unlock();
> > > > +	free_cpumask_var(enable.enabled);
> > > > +
> > > > +out:
> > > > +	return r;
> > > > +}
> > > 
> > > At last, I think there's one problem here:
> > > 
> > > KVM actually only registers CPU hotplug callback in kvm_init(), which happens
> > > way after tdx_hardware_setup().
> > > 
> > > What happens if any CPU goes online *BETWEEN* tdx_hardware_setup() and
> > > kvm_init()?
> > > 
> > > Looks we have two options:
> > > 
> > > 1) move registering CPU hotplug callback before tdx_hardware_setup(), or
> > > 2) we need to disable CPU hotplug until callbacks have been registered.

This is all so dumb (not TDX, the current state of KVM).  All of the hardware
enabling crud is pointless complex inherited from misguided, decade old paranoia
that led to the decision to enable VMX if and only if VMs are running.  Enabling
VMX doesn't make the system less secure, and the insane dances we are doing to
do VMXON on-demand makes everything *more* fragile.

And all of this complexity really was driven by VMX, enabling virtualization for
every other vendor, including AMD/SVM, is completely uninteresting.  Forcing other
architectures/vendors to take on yet more complexity doesn't make any sense.

Barely tested, and other architectures would need to be converted, but I don't
see any obvious reasons why we can't simply enable virtualization when the module
is loaded.

The diffstat pretty much says it all.

---
 Documentation/virt/kvm/locking.rst |   4 -
 arch/x86/include/asm/kvm_host.h    |   3 +
 arch/x86/kvm/svm/svm.c             |   5 +-
 arch/x86/kvm/vmx/vmx.c             |  18 ++-
 arch/x86/kvm/x86.c                 |  22 ++--
 include/linux/kvm_host.h           |   2 +
 virt/kvm/kvm_main.c                | 181 +++++++----------------------
 7 files changed, 67 insertions(+), 168 deletions(-)

diff --git a/Documentation/virt/kvm/locking.rst b/Documentation/virt/kvm/locking.rst
index 02880d5552d5..0d6eff13fd46 100644
--- a/Documentation/virt/kvm/locking.rst
+++ b/Documentation/virt/kvm/locking.rst
@@ -227,10 +227,6 @@ time it will be set using the Dirty tracking mechanism described above.
 :Type:		mutex
 :Arch:		any
 :Protects:	- vm_list
-		- kvm_usage_count
-		- hardware virtualization enable/disable
-:Comment:	KVM also disables CPU hotplug via cpus_read_lock() during
-		enable/disable.
 
 ``kvm->mn_invalidate_lock``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 73740d698ebe..7422239987d8 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -36,6 +36,7 @@
 #include <asm/kvm_page_track.h>
 #include <asm/kvm_vcpu_regs.h>
 #include <asm/hyperv-tlfs.h>
+#include <asm/reboot.h>
 
 #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
 
@@ -1605,6 +1606,8 @@ struct kvm_x86_ops {
 
 	int (*hardware_enable)(void);
 	void (*hardware_disable)(void);
+	cpu_emergency_virt_cb *emergency_disable;
+
 	void (*hardware_unsetup)(void);
 	bool (*has_emulated_msr)(struct kvm *kvm, u32 index);
 	void (*vcpu_after_set_cpuid)(struct kvm_vcpu *vcpu);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 9aaf83c8d57d..7e118284934c 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4917,6 +4917,7 @@ static void *svm_alloc_apic_backing_page(struct kvm_vcpu *vcpu)
 static struct kvm_x86_ops svm_x86_ops __initdata = {
 	.name = KBUILD_MODNAME,
 
+	.emergency_disable = svm_emergency_disable,
 	.check_processor_compatibility = svm_check_processor_compat,
 
 	.hardware_unsetup = svm_hardware_unsetup,
@@ -5348,8 +5349,6 @@ static struct kvm_x86_init_ops svm_init_ops __initdata = {
 static void __svm_exit(void)
 {
 	kvm_x86_vendor_exit();
-
-	cpu_emergency_unregister_virt_callback(svm_emergency_disable);
 }
 
 static int __init svm_init(void)
@@ -5365,8 +5364,6 @@ static int __init svm_init(void)
 	if (r)
 		return r;
 
-	cpu_emergency_register_virt_callback(svm_emergency_disable);
-
 	/*
 	 * Common KVM initialization _must_ come last, after this, /dev/kvm is
 	 * exposed to userspace!
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d18dcb1e11a6..0dbe74da7ee3 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8320,6 +8320,8 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
 
 	.hardware_enable = vmx_hardware_enable,
 	.hardware_disable = vmx_hardware_disable,
+	.emergency_disable = vmx_emergency_disable,
+
 	.has_emulated_msr = vmx_has_emulated_msr,
 
 	.vm_size = sizeof(struct kvm_vmx),
@@ -8733,8 +8735,6 @@ static void __vmx_exit(void)
 {
 	allow_smaller_maxphyaddr = false;
 
-	cpu_emergency_unregister_virt_callback(vmx_emergency_disable);
-
 	vmx_cleanup_l1d_flush();
 }
 
@@ -8760,6 +8760,12 @@ static int __init vmx_init(void)
 	 */
 	hv_init_evmcs();
 
+	for_each_possible_cpu(cpu) {
+		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
+
+		pi_init_cpu(cpu);
+	}
+
 	r = kvm_x86_vendor_init(&vmx_init_ops);
 	if (r)
 		return r;
@@ -8775,14 +8781,6 @@ static int __init vmx_init(void)
 	if (r)
 		goto err_l1d_flush;
 
-	for_each_possible_cpu(cpu) {
-		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
-
-		pi_init_cpu(cpu);
-	}
-
-	cpu_emergency_register_virt_callback(vmx_emergency_disable);
-
 	vmx_check_vmcs12_offsets();
 
 	/*
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 26288ca05364..41d3e4e32e20 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9687,15 +9687,10 @@ static int kvm_x86_check_processor_compatibility(void)
 	return static_call(kvm_x86_check_processor_compatibility)();
 }
 
-static void kvm_x86_check_cpu_compat(void *ret)
-{
-	*(int *)ret = kvm_x86_check_processor_compatibility();
-}
-
 int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 {
 	u64 host_pat;
-	int r, cpu;
+	int r;
 
 	guard(mutex)(&vendor_module_lock);
 
@@ -9771,11 +9766,11 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 
 	kvm_ops_update(ops);
 
-	for_each_online_cpu(cpu) {
-		smp_call_function_single(cpu, kvm_x86_check_cpu_compat, &r, 1);
-		if (r < 0)
-			goto out_unwind_ops;
-	}
+	cpu_emergency_register_virt_callback(kvm_x86_ops.emergency_disable);
+
+	r = kvm_enable_virtualization();
+	if (r)
+		goto out_unwind_ops;
 
 	/*
 	 * Point of no return!  DO NOT add error paths below this point unless
@@ -9818,6 +9813,7 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	return 0;
 
 out_unwind_ops:
+	cpu_emergency_unregister_virt_callback(kvm_x86_ops.emergency_disable);
 	kvm_x86_ops.hardware_enable = NULL;
 	static_call(kvm_x86_hardware_unsetup)();
 out_mmu_exit:
@@ -9858,6 +9854,10 @@ void kvm_x86_vendor_exit(void)
 	static_key_deferred_flush(&kvm_xen_enabled);
 	WARN_ON(static_branch_unlikely(&kvm_xen_enabled.key));
 #endif
+
+	kvm_disable_virtualization();
+	cpu_emergency_unregister_virt_callback(kvm_x86_ops.emergency_disable);
+
 	mutex_lock(&vendor_module_lock);
 	kvm_x86_ops.hardware_enable = NULL;
 	mutex_unlock(&vendor_module_lock);
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 48f31dcd318a..92da2eee7448 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1518,6 +1518,8 @@ static inline void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu) {}
 #endif
 
 #ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
+int kvm_enable_virtualization(void);
+void kvm_disable_virtualization(void);
 int kvm_arch_hardware_enable(void);
 void kvm_arch_hardware_disable(void);
 #endif
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index f345dc15854f..326e3225c052 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -139,8 +139,6 @@ static int kvm_no_compat_open(struct inode *inode, struct file *file)
 #define KVM_COMPAT(c)	.compat_ioctl	= kvm_no_compat_ioctl,	\
 			.open		= kvm_no_compat_open
 #endif
-static int hardware_enable_all(void);
-static void hardware_disable_all(void);
 
 static void kvm_io_bus_destroy(struct kvm_io_bus *bus);
 
@@ -1261,10 +1259,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 	if (r)
 		goto out_err_no_arch_destroy_vm;
 
-	r = hardware_enable_all();
-	if (r)
-		goto out_err_no_disable;
-
 #ifdef CONFIG_HAVE_KVM_IRQCHIP
 	INIT_HLIST_HEAD(&kvm->irq_ack_notifier_list);
 #endif
@@ -1304,8 +1298,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 		mmu_notifier_unregister(&kvm->mmu_notifier, current->mm);
 #endif
 out_err_no_mmu_notifier:
-	hardware_disable_all();
-out_err_no_disable:
 	kvm_arch_destroy_vm(kvm);
 out_err_no_arch_destroy_vm:
 	WARN_ON_ONCE(!refcount_dec_and_test(&kvm->users_count));
@@ -1393,7 +1385,6 @@ static void kvm_destroy_vm(struct kvm *kvm)
 #endif
 	kvm_arch_free_vm(kvm);
 	preempt_notifier_dec();
-	hardware_disable_all();
 	mmdrop(mm);
 }
 
@@ -5536,9 +5527,8 @@ __visible bool kvm_rebooting;
 EXPORT_SYMBOL_GPL(kvm_rebooting);
 
 static DEFINE_PER_CPU(bool, hardware_enabled);
-static int kvm_usage_count;
 
-static int __hardware_enable_nolock(void)
+static int __kvm_enable_virtualization(void)
 {
 	if (__this_cpu_read(hardware_enabled))
 		return 0;
@@ -5553,34 +5543,18 @@ static int __hardware_enable_nolock(void)
 	return 0;
 }
 
-static void hardware_enable_nolock(void *failed)
-{
-	if (__hardware_enable_nolock())
-		atomic_inc(failed);
-}
-
 static int kvm_online_cpu(unsigned int cpu)
 {
-	int ret = 0;
-
 	/*
 	 * Abort the CPU online process if hardware virtualization cannot
 	 * be enabled. Otherwise running VMs would encounter unrecoverable
 	 * errors when scheduled to this CPU.
 	 */
-	mutex_lock(&kvm_lock);
-	if (kvm_usage_count)
-		ret = __hardware_enable_nolock();
-	mutex_unlock(&kvm_lock);
-	return ret;
+	return __kvm_enable_virtualization();
 }
 
-static void hardware_disable_nolock(void *junk)
+static void __kvm_disable_virtualization(void *ign)
 {
-	/*
-	 * Note, hardware_disable_all_nolock() tells all online CPUs to disable
-	 * hardware, not just CPUs that successfully enabled hardware!
-	 */
 	if (!__this_cpu_read(hardware_enabled))
 		return;
 
@@ -5591,78 +5565,10 @@ static void hardware_disable_nolock(void *junk)
 
 static int kvm_offline_cpu(unsigned int cpu)
 {
-	mutex_lock(&kvm_lock);
-	if (kvm_usage_count)
-		hardware_disable_nolock(NULL);
-	mutex_unlock(&kvm_lock);
+	__kvm_disable_virtualization(NULL);
 	return 0;
 }
 
-static void hardware_disable_all_nolock(void)
-{
-	BUG_ON(!kvm_usage_count);
-
-	kvm_usage_count--;
-	if (!kvm_usage_count)
-		on_each_cpu(hardware_disable_nolock, NULL, 1);
-}
-
-static void hardware_disable_all(void)
-{
-	cpus_read_lock();
-	mutex_lock(&kvm_lock);
-	hardware_disable_all_nolock();
-	mutex_unlock(&kvm_lock);
-	cpus_read_unlock();
-}
-
-static int hardware_enable_all(void)
-{
-	atomic_t failed = ATOMIC_INIT(0);
-	int r;
-
-	/*
-	 * Do not enable hardware virtualization if the system is going down.
-	 * If userspace initiated a forced reboot, e.g. reboot -f, then it's
-	 * possible for an in-flight KVM_CREATE_VM to trigger hardware enabling
-	 * after kvm_reboot() is called.  Note, this relies on system_state
-	 * being set _before_ kvm_reboot(), which is why KVM uses a syscore ops
-	 * hook instead of registering a dedicated reboot notifier (the latter
-	 * runs before system_state is updated).
-	 */
-	if (system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF ||
-	    system_state == SYSTEM_RESTART)
-		return -EBUSY;
-
-	/*
-	 * When onlining a CPU, cpu_online_mask is set before kvm_online_cpu()
-	 * is called, and so on_each_cpu() between them includes the CPU that
-	 * is being onlined.  As a result, hardware_enable_nolock() may get
-	 * invoked before kvm_online_cpu(), which also enables hardware if the
-	 * usage count is non-zero.  Disable CPU hotplug to avoid attempting to
-	 * enable hardware multiple times.
-	 */
-	cpus_read_lock();
-	mutex_lock(&kvm_lock);
-
-	r = 0;
-
-	kvm_usage_count++;
-	if (kvm_usage_count == 1) {
-		on_each_cpu(hardware_enable_nolock, &failed, 1);
-
-		if (atomic_read(&failed)) {
-			hardware_disable_all_nolock();
-			r = -EBUSY;
-		}
-	}
-
-	mutex_unlock(&kvm_lock);
-	cpus_read_unlock();
-
-	return r;
-}
-
 static void kvm_shutdown(void)
 {
 	/*
@@ -5678,34 +5584,22 @@ static void kvm_shutdown(void)
 	 */
 	pr_info("kvm: exiting hardware virtualization\n");
 	kvm_rebooting = true;
-	on_each_cpu(hardware_disable_nolock, NULL, 1);
+	on_each_cpu(__kvm_disable_virtualization, NULL, 1);
 }
 
 static int kvm_suspend(void)
 {
-	/*
-	 * Secondary CPUs and CPU hotplug are disabled across the suspend/resume
-	 * callbacks, i.e. no need to acquire kvm_lock to ensure the usage count
-	 * is stable.  Assert that kvm_lock is not held to ensure the system
-	 * isn't suspended while KVM is enabling hardware.  Hardware enabling
-	 * can be preempted, but the task cannot be frozen until it has dropped
-	 * all locks (userspace tasks are frozen via a fake signal).
-	 */
-	lockdep_assert_not_held(&kvm_lock);
 	lockdep_assert_irqs_disabled();
 
-	if (kvm_usage_count)
-		hardware_disable_nolock(NULL);
+	__kvm_disable_virtualization(NULL);
 	return 0;
 }
 
 static void kvm_resume(void)
 {
-	lockdep_assert_not_held(&kvm_lock);
 	lockdep_assert_irqs_disabled();
 
-	if (kvm_usage_count)
-		WARN_ON_ONCE(__hardware_enable_nolock());
+	WARN_ON_ONCE(__kvm_enable_virtualization());
 }
 
 static struct syscore_ops kvm_syscore_ops = {
@@ -5713,16 +5607,45 @@ static struct syscore_ops kvm_syscore_ops = {
 	.resume = kvm_resume,
 	.shutdown = kvm_shutdown,
 };
-#else /* CONFIG_KVM_GENERIC_HARDWARE_ENABLING */
-static int hardware_enable_all(void)
+
+int kvm_enable_virtualization(void)
 {
+	int r;
+
+	r = cpuhp_setup_state(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
+			      kvm_online_cpu, kvm_offline_cpu);
+	if (r)
+		return r;
+
+	register_syscore_ops(&kvm_syscore_ops);
+
+	/*
+	 * Manually undo virtualization enabling if the system is going down.
+	 * If userspace initiated a forced reboot, e.g. reboot -f, then it's
+	 * possible for an in-flight module load to enable virtualization
+	 * after syscore_shutdown() is called, i.e. without kvm_shutdown()
+	 * being invoked.  Note, this relies on system_state being set _before_
+	 * kvm_shutdown(), e.g. to ensure either kvm_shutdown() is invoked
+	 * or this CPU observes the impedning shutdown.  Which is why KVM uses
+	 * a syscore ops hook instead of registering a dedicated reboot
+	 * notifier (the latter runs before system_state is updated).
+	 */
+	if (system_state == SYSTEM_HALT || system_state == SYSTEM_POWER_OFF ||
+	    system_state == SYSTEM_RESTART) {
+		unregister_syscore_ops(&kvm_syscore_ops);
+		cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
+		return -EBUSY;
+	}
+
 	return 0;
 }
 
-static void hardware_disable_all(void)
+void kvm_disable_virtualization(void)
 {
-
+	unregister_syscore_ops(&kvm_syscore_ops);
+	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
 }
+
 #endif /* CONFIG_KVM_GENERIC_HARDWARE_ENABLING */
 
 static void kvm_iodevice_destructor(struct kvm_io_device *dev)
@@ -6418,15 +6341,6 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 	int r;
 	int cpu;
 
-#ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
-	r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
-				      kvm_online_cpu, kvm_offline_cpu);
-	if (r)
-		return r;
-
-	register_syscore_ops(&kvm_syscore_ops);
-#endif
-
 	/* A kmem cache lets us meet the alignment requirements of fx_save. */
 	if (!vcpu_align)
 		vcpu_align = __alignof__(struct kvm_vcpu);
@@ -6437,10 +6351,8 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 					   offsetofend(struct kvm_vcpu, stats_id)
 					   - offsetof(struct kvm_vcpu, arch),
 					   NULL);
-	if (!kvm_vcpu_cache) {
-		r = -ENOMEM;
-		goto err_vcpu_cache;
-	}
+	if (!kvm_vcpu_cache)
+		return -ENOMEM;
 
 	for_each_possible_cpu(cpu) {
 		if (!alloc_cpumask_var_node(&per_cpu(cpu_kick_mask, cpu),
@@ -6497,11 +6409,6 @@ int kvm_init(unsigned vcpu_size, unsigned vcpu_align, struct module *module)
 	for_each_possible_cpu(cpu)
 		free_cpumask_var(per_cpu(cpu_kick_mask, cpu));
 	kmem_cache_destroy(kvm_vcpu_cache);
-err_vcpu_cache:
-#ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
-	unregister_syscore_ops(&kvm_syscore_ops);
-	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_ONLINE);
-#endif
 	return r;
 }
 EXPORT_SYMBOL_GPL(kvm_init);
@@ -6523,10 +6430,6 @@ void kvm_exit(void)
 	kmem_cache_destroy(kvm_vcpu_cache);
 	kvm_vfio_ops_exit();
 	kvm_async_pf_deinit();
-#ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
-	unregister_syscore_ops(&kvm_syscore_ops);
-	cpuhp_remove_state_nocalls(CPUHP_AP_KVM_ONLINE);
-#endif
 	kvm_irqfd_exit();
 }
 EXPORT_SYMBOL_GPL(kvm_exit);

base-commit: f10f3621ad80f008c218dbbc13a05c893766a7d2
--