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Subject: Re: [PATCH 1/2] s390/vfio-ap: r/w lock for PQAP interception handler
 function pointer
To:     Tony Krowiak <akrowiak@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     cohuck@redhat.com, pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
        jgg@nvidia.com, alex.williamson@redhat.com, kwankhede@nvidia.com,
        david@redhat.com
References: <20210719193503.793910-1-akrowiak@linux.ibm.com>
 <20210719193503.793910-2-akrowiak@linux.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <1a9f15d7-0f4d-00a0-0a8b-f1c08aa52eeb@de.ibm.com>
Date:   Wed, 18 Aug 2021 19:03:33 +0200
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On 19.07.21 21:35, Tony Krowiak wrote:
> The function pointer to the interception handler for the PQAP instruction
> can get changed during the interception process. Let's add a
> semaphore to struct kvm_s390_crypto to control read/write access to the
> function pointer contained therein.
> 
> The semaphore must be locked for write access by the vfio_ap device driver
> when notified that the KVM pointer has been set or cleared. It must be
> locked for read access by the interception framework when the PQAP
> instruction is intercepted.
> 
> Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   arch/s390/include/asm/kvm_host.h      |  8 +++-----
>   arch/s390/kvm/kvm-s390.c              |  1 +
>   arch/s390/kvm/priv.c                  | 10 ++++++----
>   drivers/s390/crypto/vfio_ap_ops.c     | 23 +++++++++++++++++------
>   drivers/s390/crypto/vfio_ap_private.h |  2 +-
>   5 files changed, 28 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index 9b4473f76e56..f18849d259e6 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -798,14 +798,12 @@ struct kvm_s390_cpu_model {
>   	unsigned short ibc;
>   };
>   
> -struct kvm_s390_module_hook {
> -	int (*hook)(struct kvm_vcpu *vcpu);
> -	struct module *owner;
> -};
> +typedef int (*crypto_hook)(struct kvm_vcpu *vcpu);
>   
>   struct kvm_s390_crypto {
>   	struct kvm_s390_crypto_cb *crycb;
> -	struct kvm_s390_module_hook *pqap_hook;
> +	struct rw_semaphore pqap_hook_rwsem;
> +	crypto_hook *pqap_hook;
>   	__u32 crycbd;
>   	__u8 aes_kw;
>   	__u8 dea_kw;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index b655a7d82bf0..a08f242a9f27 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2630,6 +2630,7 @@ static void kvm_s390_crypto_init(struct kvm *kvm)
>   {
>   	kvm->arch.crypto.crycb = &kvm->arch.sie_page2->crycb;
>   	kvm_s390_set_crycb_format(kvm);
> +	init_rwsem(&kvm->arch.crypto.pqap_hook_rwsem);
>   
>   	if (!test_kvm_facility(kvm, 76))
>   		return;
> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> index 9928f785c677..6bed9406c1f3 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -610,6 +610,7 @@ static int handle_io_inst(struct kvm_vcpu *vcpu)
>   static int handle_pqap(struct kvm_vcpu *vcpu)
>   {
>   	struct ap_queue_status status = {};
> +	crypto_hook pqap_hook;
>   	unsigned long reg0;
>   	int ret;
>   	uint8_t fc;
> @@ -657,15 +658,16 @@ static int handle_pqap(struct kvm_vcpu *vcpu)
>   	 * Verify that the hook callback is registered, lock the owner
>   	 * and call the hook.
>   	 */
> +	down_read(&vcpu->kvm->arch.crypto.pqap_hook_rwsem);
>   	if (vcpu->kvm->arch.crypto.pqap_hook) {
> -		if (!try_module_get(vcpu->kvm->arch.crypto.pqap_hook->owner))
> -			return -EOPNOTSUPP;
> -		ret = vcpu->kvm->arch.crypto.pqap_hook->hook(vcpu);
> -		module_put(vcpu->kvm->arch.crypto.pqap_hook->owner);
> +		pqap_hook = *vcpu->kvm->arch.crypto.pqap_hook;

Dont we have to check for NULL here? If not can you add a comment why?

Otherwise this looks good.


> +		ret = pqap_hook(vcpu);
[...]