linux-kernel - Re: [PATCH 3/6] svm: Add support for APIC_ACCESS_PAGE_PRIVATE

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Date:   Wed, 8 May 2019 21:14:43 +0200
From:   Jan H. Schönherr <jschoenh@...zon.de>
To:     "Suthikulpanit, Suravee" <Suravee.Suthikulpanit@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "joro@...tes.org" <joro@...tes.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH 3/6] svm: Add support for APIC_ACCESS_PAGE_PRIVATE_MEMSLOT
 setup/destroy

On 22/03/2019 12.57, Suthikulpanit, Suravee wrote:
> Activate/deactivate AVIC requires setting/unsetting the memory region used
> for APIC_ACCESS_PAGE_PRIVATE_MEMSLOT. So, re-factor avic_init_access_page()
> to avic_setup_access_page() and add srcu_read_lock/unlock, which are needed
> to allow this function to be called during run-time.
> 
> Also, introduce avic_destroy_access_page() to unset the page when
> deactivate AVIC.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
>  arch/x86/kvm/svm.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 4cf93a729ad8..f41f34f70dde 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1666,7 +1666,7 @@ static u64 *avic_get_physical_id_entry(struct kvm_vcpu *vcpu,
>   * field of the VMCB. Therefore, we set up the
>   * APIC_ACCESS_PAGE_PRIVATE_MEMSLOT (4KB) here.
>   */
> -static int avic_init_access_page(struct kvm_vcpu *vcpu)
> +static int avic_setup_access_page(struct kvm_vcpu *vcpu, bool init)
>  {
>  	struct kvm *kvm = vcpu->kvm;
>  	int ret = 0;
> @@ -1675,10 +1675,14 @@ static int avic_init_access_page(struct kvm_vcpu *vcpu)
>  	if (kvm->arch.apic_access_page_done)
>  		goto out;
>  
> +	if (!init)
> +		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
>  	ret = __x86_set_memory_region(kvm,
>  				      APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
>  				      APIC_DEFAULT_PHYS_BASE,
>  				      PAGE_SIZE);
> +	if (!init)
> +		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
>  	if (ret)
>  		goto out;
>  
> @@ -1688,6 +1692,26 @@ static int avic_init_access_page(struct kvm_vcpu *vcpu)
>  	return ret;
>  }
>  
> +static void avic_destroy_access_page(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm *kvm = vcpu->kvm;
> +
> +	mutex_lock(&kvm->slots_lock);
> +
> +	if (!kvm->arch.apic_access_page_done)
> +		goto out;
> +
> +	srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> +	__x86_set_memory_region(kvm,
> +				APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
> +				APIC_DEFAULT_PHYS_BASE,
> +				0);
> +	vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);

This pattern of "unlock, do something, re-lock" strikes me as odd --
here and in the setup function.

There seem to be a few assumptions for this to work:
a) SRCU read-side critical sections must not be nested.
b) We must not keep any pointer to a SRCU protected structure
   across a call to this function.

Can we guarantee these assumptions? Now and in the future (given that this is already
a bit hidden in the call stack)?

(And if we can guarantee them, why are we holding the SRCU lock in the first place?)

Or is there maybe a nicer way to do this?

Regards
Jan

> +	kvm->arch.apic_access_page_done = false;
> +out:
> +	mutex_unlock(&kvm->slots_lock);
> +}
> +
>  static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  {
>  	int ret;
> @@ -1695,7 +1719,7 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  	int id = vcpu->vcpu_id;
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	ret = avic_init_access_page(vcpu);
> +	ret = avic_setup_access_page(vcpu, true);
>  	if (ret)
>  		return ret;
>  
>