linux-kernel - Re: [PATCH v2] KVM: x86/xen: Take srcu lock when accessing kvm

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Message-ID: <YILlPmN0fgLA8RkJ@google.com>
Date:   Fri, 23 Apr 2021 15:18:22 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2] KVM: x86/xen: Take srcu lock when accessing
 kvm_memslots()

On Fri, Apr 23, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> kvm_memslots() will be called by kvm_write_guest_offset_cached() so we should 
> take the srcu lock. Let's pull the srcu lock operation from kvm_steal_time_set_preempted() 
> again to fix xen part.
> 
> Fixes: 30b5c851af7 (KVM: x86/xen: Add support for vCPU runstate information)
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/x86.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3bf52ba..c775d24 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4097,7 +4097,6 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_host_map map;
>  	struct kvm_steal_time *st;
> -	int idx;
>  
>  	if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
>  		return;
> @@ -4105,15 +4104,9 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>  	if (vcpu->arch.st.preempted)
>  		return;
>  
> -	/*
> -	 * Take the srcu lock as memslots will be accessed to check the gfn
> -	 * cache generation against the memslots generation.
> -	 */
> -	idx = srcu_read_lock(&vcpu->kvm->srcu);
> -
>  	if (kvm_map_gfn(vcpu, vcpu->arch.st.msr_val >> PAGE_SHIFT, &map,
>  			&vcpu->arch.st.cache, true))
> -		goto out;
> +		return;
>  
>  	st = map.hva +
>  		offset_in_page(vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS);
> @@ -4121,20 +4114,25 @@ static void kvm_steal_time_set_preempted(struct kvm_vcpu *vcpu)
>  	st->preempted = vcpu->arch.st.preempted = KVM_VCPU_PREEMPTED;
>  
>  	kvm_unmap_gfn(vcpu, &map, &vcpu->arch.st.cache, true, true);
> -
> -out:
> -	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  }
>  
>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
> +	int idx;
> +
>  	if (vcpu->preempted && !vcpu->arch.guest_state_protected)
>  		vcpu->arch.preempted_in_kernel = !static_call(kvm_x86_get_cpl)(vcpu);
>  
> +	/*
> +	 * Take the srcu lock as memslots will be accessed to check the gfn
> +	 * cache generation against the memslots generation.
> +	 */
> +	idx = srcu_read_lock(&vcpu->kvm->srcu);

Might be worth grabbing "kvm" in a local variable?  Either way:

Reviewed-by: Sean Christopherson <seanjc@...gle.com>

>  	if (kvm_xen_msr_enabled(vcpu->kvm))
>  		kvm_xen_runstate_set_preempted(vcpu);
>  	else
>  		kvm_steal_time_set_preempted(vcpu);
> +	srcu_read_unlock(&vcpu->kvm->srcu, idx);
>  
>  	static_call(kvm_x86_vcpu_put)(vcpu);
>  	vcpu->arch.last_host_tsc = rdtsc();
> -- 
> 2.7.4
>