linux-kernel - Re: [PATCH v7 10/17] KVM: s390: pv: add mmu

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Date:   Mon, 7 Feb 2022 12:04:56 +0100
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, thuth@...hat.com, pasic@...ux.ibm.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com
Subject: Re: [PATCH v7 10/17] KVM: s390: pv: add mmu_notifier

On 2/4/22 16:53, Claudio Imbrenda wrote:
> Add an mmu_notifier for protected VMs. The callback function is
> triggered when the mm is torn down, and will attempt to convert all
> protected vCPUs to non-protected. This allows the mm teardown to use
> the destroy page UVC instead of export.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> ---
>   arch/s390/include/asm/kvm_host.h |  2 ++
>   arch/s390/kvm/pv.c               | 20 ++++++++++++++++++++
>   2 files changed, 22 insertions(+)
> 
> diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
> index a22c9266ea05..1bccb8561ba9 100644
> --- a/arch/s390/include/asm/kvm_host.h
> +++ b/arch/s390/include/asm/kvm_host.h
> @@ -19,6 +19,7 @@
>   #include <linux/kvm.h>
>   #include <linux/seqlock.h>
>   #include <linux/module.h>
> +#include <linux/mmu_notifier.h>
>   #include <asm/debug.h>
>   #include <asm/cpu.h>
>   #include <asm/fpu/api.h>
> @@ -921,6 +922,7 @@ struct kvm_s390_pv {
>   	u64 guest_len;
>   	unsigned long stor_base;
>   	void *stor_var;
> +	struct mmu_notifier mmu_notifier;
>   };
>   
>   struct kvm_arch{
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index f1e812a45acb..d3b8fd9b5b3e 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -193,6 +193,21 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>   	return -EIO;
>   }
>   
> +static void kvm_s390_pv_mmu_notifier_release(struct mmu_notifier *subscription,
> +					     struct mm_struct *mm)
> +{
> +	struct kvm *kvm = container_of(subscription, struct kvm, arch.pv.mmu_notifier);

Are we sure that the kvm pointer is still valid at this point?

> +	u16 dummy;
> +
> +	mutex_lock(&kvm->lock);

Against what are we locking here?

> +	kvm_s390_cpus_from_pv(kvm, &dummy, &dummy);

I'd guess that we can't really have a second kvm_s390_cpus_from_pv() 
call in flight, right? If the mm is being torn down there would be no 
code left that can execute the IOCTL.

> +	mutex_unlock(&kvm->lock);
> +}
> +
> +static const struct mmu_notifier_ops kvm_s390_pv_mmu_notifier_ops = {
> +	.release = kvm_s390_pv_mmu_notifier_release,
> +};
> +
>   int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>   {
>   	struct uv_cb_cgc uvcb = {
> @@ -234,6 +249,11 @@ int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>   		return -EIO;
>   	}
>   	kvm->arch.gmap->guest_handle = uvcb.guest_handle;
> +	/* Add the notifier only once. No races because we hold kvm->lock */
> +	if (kvm->arch.pv.mmu_notifier.ops != &kvm_s390_pv_mmu_notifier_ops) {
> +		kvm->arch.pv.mmu_notifier.ops = &kvm_s390_pv_mmu_notifier_ops;
> +		mmu_notifier_register(&kvm->arch.pv.mmu_notifier, kvm->mm);
> +	}
>   	return 0;
>   }
>   
>