Re: [PATCH v3 5/8] KVM: Add arch hooks for enabling/disabling virtualization

[Paolo Bonzini]

On 6/8/24 02:06, Sean Christopherson wrote:
> Add arch hooks that are invoked when KVM enables/disable virtualization.
> x86 will use the hooks to register an "emergency disable" callback, which
> is essentially an x86-specific shutdown notifier that is used when the
> kernel is doing an emergency reboot/shutdown/kexec.
>
> Add comments for the declarations to help arch code understand exactly
> when the callbacks are invoked.  Alternatively, the APIs themselves could
> communicate most of the same info, but kvm_arch_pre_enable_virtualization()
> and kvm_arch_post_disable_virtualization() are a bit cumbersome, and make
> it a bit less obvious that they are intended to be implemented as a pair.
>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   include/linux/kvm_host.h | 14 ++++++++++++++
>   virt/kvm/kvm_main.c      | 14 ++++++++++++++
>   2 files changed, 28 insertions(+)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 96ad3e8b9ddb..12ef3beb4e47 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -1514,6 +1514,20 @@ static inline void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu) {}
>   #endif
>   
>   #ifdef CONFIG_KVM_GENERIC_HARDWARE_ENABLING
> +/*
> + * kvm_arch_{enable,disable}_virtualization() are called on one CPU, under
> + * kvm_usage_lock, immediately after/before 0=>1 and 1=>0 transitions of
> + * kvm_usage_count, i.e. at the beginning of the generic hardware enabling
> + * sequence, and at the end of the generic hardware disabling sequence.
> + */
> +void kvm_arch_enable_virtualization(void);
> +void kvm_arch_disable_virtualization(void);
> +/*
> + * kvm_arch_hardware_{enable,disable}() are called on "every" CPU to do the
> + * actual twiddling of hardware bits.  The hooks are called all online CPUs
> + * when KVM enables/disabled virtualization.  Enabling/disabling is also done
> + * when a CPU is onlined/offlined (or Resumed/Suspended).
> + */
>   int kvm_arch_hardware_enable(void);
>   void kvm_arch_hardware_disable(void);

Since you are at it, rename these to 
kvm_arch_{enable,disable}_virtualization_cpu()?

Paolo

>   #endif
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 7bdd744e4821..e20189a89a64 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -5505,6 +5505,16 @@ static DEFINE_PER_CPU(bool, hardware_enabled);
>   static DEFINE_MUTEX(kvm_usage_lock);
>   static int kvm_usage_count;
>   
> +__weak void kvm_arch_enable_virtualization(void)
> +{
> +
> +}
> +
> +__weak void kvm_arch_disable_virtualization(void)
> +{
> +
> +}
> +
>   static int __kvm_enable_virtualization(void)
>   {
>   	if (__this_cpu_read(hardware_enabled))
> @@ -5604,6 +5614,8 @@ static int kvm_enable_virtualization(void)
>   	if (kvm_usage_count++)
>   		return 0;
>   
> +	kvm_arch_enable_virtualization();
> +
>   	r = cpuhp_setup_state(CPUHP_AP_KVM_ONLINE, "kvm/cpu:online",
>   			      kvm_online_cpu, kvm_offline_cpu);
>   	if (r)
> @@ -5634,6 +5646,7 @@ static int kvm_enable_virtualization(void)
>   	unregister_syscore_ops(&kvm_syscore_ops);
>   	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
>   err_cpuhp:
> +	kvm_arch_disable_virtualization();
>   	--kvm_usage_count;
>   	return r;
>   }
> @@ -5647,6 +5660,7 @@ static void kvm_disable_virtualization(void)
>   
>   	unregister_syscore_ops(&kvm_syscore_ops);
>   	cpuhp_remove_state(CPUHP_AP_KVM_ONLINE);
> +	kvm_arch_disable_virtualization();
>   }
>   
>   static int kvm_init_virtualization(void)