linux-kernel - Re: [RFC PATCH v6 003/104] KVM: Refactor CPU compatibility check on module initialiization

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Date:   Thu, 26 May 2022 12:03:55 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sagi Shahar <sagis@...gle.com>
Cc:     "Yamahata, Isaku" <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v6 003/104] KVM: Refactor CPU compatibility check on
 module initialiization

On Mon, May 23, 2022 at 03:27:10PM -0700,
Sagi Shahar <sagis@...gle.com> wrote:

> On Thu, May 5, 2022 at 11:15 AM <isaku.yamahata@...el.com> wrote:
> >
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> >
> > Although non-x86 arch doesn't break as long as I inspected code, it's by
> > code inspection.  This should be reviewed by each arch maintainers.
> >
> > kvm_init() checks CPU compatibility by calling
> > kvm_arch_check_processor_compat() on all online CPUs.  Move the callback
> > to hardware_enable_nolock() and add hardware_enable_all() and
> > hardware_disable_all().
> > Add arch specific callback kvm_arch_post_hardware_enable_setup() for arch
> > to do arch specific initialization that required hardware_enable_all().
> 
> There's no reference to kvm_arch_post_hardware_enable_setup in this
> patch. Looks like it's introduced in a later patch in the series.
> 
> This patch might be clearer if the kvm_arch_post_hardware_enable_setup
> is introduced and used here. Otherwise, the commit log should be
> updated to make it clear that kvm_arch_post_hardware_enable_setup is
> introduced in a later patch in the series.
> 
> > This makes a room for TDX module to initialize on kvm module loading.  TDX
> > module requires all online cpu to enable VMX by VMXON.
> >
> > If kvm_arch_hardware_enable/disable() depend on (*) part, such dependency
> > must be called before kvm_init().  In fact kvm_intel() does.  Although
> > other arch doesn't as long as I checked as follows, it should be reviewed
> > by each arch maintainers.

Thank you for pointing it out. I moved the addition of
kvm_arch_post_hardware_enable_setup() to this patch. The related patch is as
follows.

The patch that actually uses kvm_arch_post_hardware_enable_setup() is
"011/104 KVM: TDX: Initialize TDX module when loading kvm_intel.ko".  It's
a bit far from this patch.  I intentionally positioned this patch early in
this series because this patch requires cross-arch review.

Thanks,

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 1aead3921a16..55dd08cca5d2 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1441,6 +1441,7 @@ void kvm_arch_create_vcpu_debugfs(struct kvm_vcpu *vcpu, struct dentry *debugfs_
 int kvm_arch_hardware_enable(void);
 void kvm_arch_hardware_disable(void);
 int kvm_arch_hardware_setup(void *opaque);
+int kvm_arch_post_hardware_enable_setup(void *opaque);
 void kvm_arch_hardware_unsetup(void);
 int kvm_arch_check_processor_compat(void);
 int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ec365291c625..55b292fda733 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4883,8 +4883,13 @@ static void hardware_enable_nolock(void *junk)
 
 	cpumask_set_cpu(cpu, cpus_hardware_enabled);
 
+	r = kvm_arch_check_processor_compat();
+	if (r)
+		goto out;
+
 	r = kvm_arch_hardware_enable();
 
+out:
 	if (r) {
 		cpumask_clear_cpu(cpu, cpus_hardware_enabled);
 		atomic_inc(&hardware_enable_failed);
@@ -5681,9 +5686,9 @@ void kvm_unregister_perf_callbacks(void)
 }
 #endif
 
-static void check_processor_compat(void *rtn)
+__weak int kvm_arch_post_hardware_enable_setup(void *opaque)
 {
-	*(int *)rtn = kvm_arch_check_processor_compat();
+	return 0;
 }
 
 int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
@@ -5716,11 +5721,17 @@ int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
 	if (r < 0)
 		goto out_free_1;
 
-	for_each_online_cpu(cpu) {
-		smp_call_function_single(cpu, check_processor_compat, &r, 1);
-		if (r < 0)
-			goto out_free_2;
-	}
+	/* hardware_enable_nolock() checks CPU compatibility on each CPUs. */
+	r = hardware_enable_all();
+	if (r)
+		goto out_free_2;
+	/*
+	 * Arch specific initialization that requires to enable virtualization
+	 * feature.  e.g. TDX module initialization requires VMXON on all
+	 * present CPUs.
+	 */
+	kvm_arch_post_hardware_enable_setup(opaque);
+	hardware_disable_all();
 
 	r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_STARTING, "kvm/cpu:starting",
 				      kvm_starting_cpu, kvm_dying_cpu);


-- 
Isaku Yamahata <isaku.yamahata@...il.com>