Date:   Wed, 13 Jul 2022 15:11:07 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     isaku.yamahata@...el.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v7 003/102] KVM: Refactor CPU compatibility check on
 module initialiization

On Mon, 2022-06-27 at 14:52 -0700, 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.

This first paragraph doesn't make sense to me.  At this moment, we don't know
why you need this patch at all.

> 
> kvm_init() checks CPU compatibility by calling
> kvm_arch_check_processor_compat() on all online CPUs.  
> 

What's the problem here which requires you need to do ..

> Move the callback
> to hardware_enable_nolock() and add hardware_enable_all() and
> hardware_disable_all().

.. this?


> Add arch specific callback kvm_arch_post_hardware_enable_setup() for arch
> to do arch specific initialization that required hardware_enable_all().
> This makes a room for TDX module to initialize on kvm module loading.  TDX
> module requires all online cpu to enable VMX by VMXON.

So to me this is the reason why you need to do hardware_enable_all() in
kvm_init().  There's nothing wrong with "kvm_init() checks CPU compatibility by
calling kvm_arch_check_processor_compat() on all online CPUs", right?

In this case, shouldn't we say something like "opportunistically move
kvm_arch_check_processor_compat() to hardware_enable_nolock()" because this is
not the reason that you want this patch, correct?

Also, maybe I am missing something obviously, but why do you need to do
hardware_disable_all() right after hardware_enable_all() in kvm_init()?  Could
you at least put some explanation in the changelog?

And again, it's better to add one sentence or so to explain why do you want to
init TDX module during module loading time.

> 
> If kvm_arch_hardware_enable/disable() depend on (*) part, such dependency
> must be called before kvm_init().  
> 

I don't follow the logic here.  If kvm_arch_hardware_enable() depends on
something, then you need to put kvm_arch_hardware_enable() after that, or move
that forward.  But why such dependency must be called "before kvm_init()"?

Also, I think you are talking about the problem that _after_ you move
hardware_enable_all() to kvm_init(), but not problem in existing code, right?


> In fact kvm_intel() does.  
> 

No such function kvm_intel().

Again, what's the issue here? Can you add more sentences to explain the
_problem_, or _why_?


> Although
> other arch doesn't as long as I checked as follows, it should be reviewed
> by each arch maintainers.
> 
> Before this patch:
> - Arch module initialization
>   - kvm_init()
>     - kvm_arch_init()
>     - kvm_arch_check_processor_compat() on each CPUs
>   - post arch specific initialization ---- (*)
> 
> - when creating/deleting first/last VM
>    - kvm_arch_hardware_enable() on each CPUs --- (A)
>    - kvm_arch_hardware_disable() on each CPUs --- (B)
> 
> After this patch:
> - Arch module initialization
>   - kvm_init()
>     - kvm_arch_init()
>     - kvm_arch_hardware_enable() on each CPUs  (A)
>     - kvm_arch_check_processor_compat() on each CPUs

Even with this patch, unless I am seeing mistakenly, kvm_arch_hardware_enable()
is called _after_ kvm_arch_check_processor_compat().

>     - kvm_arch_hardware_disable() on each CPUs (B)
>   - post arch specific initialization  --- (*)
> 
> Code inspection result:
> (A)/(B) can depend on (*) before this patch.  If there is dependency, such
> initialization must be moved before kvm_init() with this patch.  
> 

Must be moved to before (A)/(B), right?

> VMX does
> in fact.  
> 

More details will help, please.

> As long as I inspected other archs and find only mips has it.
> 
> - arch/mips/kvm/mips.c
>   module init function, kvm_mips_init(), does some initialization after
>   kvm_init().  Compile test only.  Needs review.

Is "Needs review" changelog material?

> 
> - arch/x86/kvm/x86.c
>   - uses vm_list which is statically initialized.

I can hardly see how "vm_list is statically initialized" is causing any problem
here.  Exactly what's the problem here??

>   - static_call(kvm_x86_hardware_enable)();
>     - SVM: (*) is empty.
>     - VMX: needs fix

What's the problem, and how are you going to fix?  Shouldn't this be in
changelog?

> 
> - arch/powerpc/kvm/powerpc.c
>   kvm_arch_hardware_enable/disable() are nop
> 
> - arch/s390/kvm/kvm-s390.c
>   kvm_arch_hardware_enable/disable() are nop
> 
> - arch/arm64/kvm/arm.c
>   module init function, arm_init(), calls only kvm_init().
>   (*) is empty
> 
> - arch/riscv/kvm/main.c
>   module init function, riscv_kvm_init(), calls only kvm_init().
>   (*) is empty
> 
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> ---
>  arch/mips/kvm/mips.c     | 12 +++++++-----
>  arch/x86/kvm/vmx/vmx.c   | 15 +++++++++++----
>  include/linux/kvm_host.h |  1 +
>  virt/kvm/kvm_main.c      | 25 ++++++++++++++++++-------
>  4 files changed, 37 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/mips/kvm/mips.c b/arch/mips/kvm/mips.c
> index 092d09fb6a7e..17228584485d 100644
> --- a/arch/mips/kvm/mips.c
> +++ b/arch/mips/kvm/mips.c
> @@ -1643,11 +1643,6 @@ static int __init kvm_mips_init(void)
>  	}
>  
>  	ret = kvm_mips_entry_setup();
> -	if (ret)
> -		return ret;
> -
> -	ret = kvm_init(NULL, sizeof(struct kvm_vcpu), 0, THIS_MODULE);
> -
>  	if (ret)
>  		return ret;
>  
> @@ -1656,6 +1651,13 @@ static int __init kvm_mips_init(void)
>  
>  	register_die_notifier(&kvm_mips_csr_die_notifier);
>  
> +	ret = kvm_init(NULL, sizeof(struct kvm_vcpu), 0, THIS_MODULE);
> +
> +	if (ret) {
> +		unregister_die_notifier(&kvm_mips_csr_die_notifier);
> +		return ret;
> +	}
> +

I don't understand how moving "hardware_enable_all()/hardware_disable_all()" to
kvm_init() is related to this change.

Anyway, at least some comments?

>  	return 0;
>  }
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 31e7630203fd..d3b68a6dec48 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8372,6 +8372,15 @@ static void vmx_exit(void)
>  }
>  module_exit(vmx_exit);
>  
> +/* initialize before kvm_init() so that hardware_enable/disable() can work. */

There's no function named hardware_enable() or hardware_disable().

> +static void __init vmx_init_early(void)
> +{
> +	int cpu;
> +
> +	for_each_possible_cpu(cpu)
> +		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> +}
> +

Perhaps I am missing something, but I couldn't see why this must be done before
kvm_init().  Please give some comments?

>  static int __init vmx_init(void)
>  {
>  	int r, cpu;
> @@ -8409,6 +8418,7 @@ static int __init vmx_init(void)
>  	}
>  #endif
>  
> +	vmx_init_early();
>  	r = kvm_init(&vmx_init_ops, sizeof(struct vcpu_vmx),
>  		     __alignof__(struct vcpu_vmx), THIS_MODULE);
>  	if (r)
> @@ -8427,11 +8437,8 @@ static int __init vmx_init(void)
>  		return r;
>  	}
>  
> -	for_each_possible_cpu(cpu) {
> -		INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> -
> +	for_each_possible_cpu(cpu)
>  		pi_init_cpu(cpu);
> -	}
>  
>  #ifdef CONFIG_KEXEC_CORE
>  	rcu_assign_pointer(crash_vmclear_loaded_vmcss,
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index d4f130a9f5c8..79a4988fd51f 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 a5bada53f1fe..cee799265ce6 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -4899,8 +4899,13 @@ 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);
> @@ -5697,9 +5702,9 @@ 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,
> @@ -5732,11 +5737,17 @@ 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);

So after digging history and looking at the code again, I guess perhaps it's
also fine to introduce this __weak version here (since you have given the reason
to do so in changelog), but in this way perhaps it's better to put this patch
and the patch to load TDX module closely so it's easier to review.

Or to me it's also fine to move this chunk to the patch to init TDX module as I
replied before.

> +	hardware_disable_all();

IMO needs a comment why do we need to do hardware_disable_all() here.  It
doesn't make a lot sense to me.

>  
>  	r = cpuhp_setup_state_nocalls(CPUHP_AP_KVM_STARTING, "kvm/cpu:starting",
>  				      kvm_starting_cpu, kvm_dying_cpu);

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