linux-kernel - Re: [PATCH 1/3] KVM: VMX: Always VMCLEAR in-use VMCSes during crash with kexec support

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Date:   Fri, 28 Feb 2020 06:59:43 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] KVM: VMX: Always VMCLEAR in-use VMCSes during crash
 with kexec support

On Fri, Feb 28, 2020 at 11:16:10AM +0100, Paolo Bonzini wrote:
> On 27/02/20 23:30, Sean Christopherson wrote:
> > -void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs)
> > +void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs, bool in_use)
> >  {
> >  	vmcs_clear(loaded_vmcs->vmcs);
> >  	if (loaded_vmcs->shadow_vmcs && loaded_vmcs->launched)
> >  		vmcs_clear(loaded_vmcs->shadow_vmcs);
> > +
> > +	if (in_use) {
> > +		list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
> > +
> > +		/*
> > +		 * Ensure deleting loaded_vmcs from its current percpu list
> > +		 * completes before setting loaded_vmcs->vcpu to -1, otherwise
> > +		 * a different cpu can see vcpu == -1 first and add loaded_vmcs
> > +		 * to its percpu list before it's deleted from this cpu's list.
> > +		 * Pairs with the smp_rmb() in vmx_vcpu_load_vmcs().
> > +		 */
> > +		smp_wmb();
> > +	}
> > +
> 
> I'd like to avoid the new in_use argument and, also, I think it's a
> little bit nicer to always invoke the memory barrier.  Even though we 
> use "asm volatile" for vmclear and therefore the compiler is already 
> taken care of, in principle it's more correct to order the ->cpu write 
> against vmclear's.

Completely agree on all points.  I wanted to avoid in_use as well, but it
didn't occur to me to use list_empty()...

> This gives the following patch on top:

Looks good.

> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c9d6152e7a4d..77a64110577b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -656,25 +656,24 @@ static int vmx_set_guest_msr(struct vcpu_vmx *vmx, struct shared_msr_entry *msr,
>  	return ret;
>  }
>  
> -void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs, bool in_use)
> +void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs)
>  {
>  	vmcs_clear(loaded_vmcs->vmcs);
>  	if (loaded_vmcs->shadow_vmcs && loaded_vmcs->launched)
>  		vmcs_clear(loaded_vmcs->shadow_vmcs);
>  
> -	if (in_use) {
> +	if (!list_empty(&loaded_vmcs->loaded_vmcss_on_cpu_link))
>  		list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
>  
> -		/*
> -		 * Ensure deleting loaded_vmcs from its current percpu list
> -		 * completes before setting loaded_vmcs->vcpu to -1, otherwise
> -		 * a different cpu can see vcpu == -1 first and add loaded_vmcs
> -		 * to its percpu list before it's deleted from this cpu's list.
> -		 * Pairs with the smp_rmb() in vmx_vcpu_load_vmcs().
> -		 */
> -		smp_wmb();
> -	}
> -
> +	/*
> +	 * Ensure all writes to loaded_vmcs, including deleting it
> +	 * from its current percpu list, complete before setting
> +	 * loaded_vmcs->vcpu to -1; otherwise,, a different cpu can
> +	 * see vcpu == -1 first and add loaded_vmcs to its percpu
> +	 * list before it's deleted from this cpu's list.  Pairs
> +	 * with the smp_rmb() in vmx_vcpu_load_vmcs().
> +	 */
> +	smp_wmb();
>  	loaded_vmcs->cpu = -1;
>  	loaded_vmcs->launched = 0;
>  }
> @@ -701,7 +700,7 @@ static void __loaded_vmcs_clear(void *arg)
>  	if (per_cpu(current_vmcs, cpu) == loaded_vmcs->vmcs)
>  		per_cpu(current_vmcs, cpu) = NULL;
>  
> -	loaded_vmcs_init(loaded_vmcs, true);
> +	loaded_vmcs_init(loaded_vmcs);
>  }
>  
>  void loaded_vmcs_clear(struct loaded_vmcs *loaded_vmcs)
> @@ -2568,7 +2567,8 @@ int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>  
>  	loaded_vmcs->shadow_vmcs = NULL;
>  	loaded_vmcs->hv_timer_soft_disabled = false;
> -	loaded_vmcs_init(loaded_vmcs, false);
> +	INIT_LIST_HEAD(&loaded_vmcs->loaded_vmcss_on_cpu_link);
> +	loaded_vmcs_init(loaded_vmcs);
>  
>  	if (cpu_has_vmx_msr_bitmap()) {
>  		loaded_vmcs->msr_bitmap = (unsigned long *)
> 
> Paolo
>