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Date:   Fri, 13 Jan 2023 22:50:22 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Chao Peng <chao.p.peng@linux.intel.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
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        Naoya Horiguchi <naoya.horiguchi@nec.com>,
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Subject: Re: [PATCH v10 6/9] KVM: Unmap existing mappings when change the
 memory attributes
Message-ID: <Y8HgLq/CqTaEi/ME@google.com>
References: <20221202061347.1070246-1-chao.p.peng@linux.intel.com>
 <20221202061347.1070246-7-chao.p.peng@linux.intel.com>
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In-Reply-To: <20221202061347.1070246-7-chao.p.peng@linux.intel.com>
Precedence: bulk

On Fri, Dec 02, 2022, Chao Peng wrote:
> @@ -785,11 +786,12 @@ struct kvm {
>  
>  #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
>  	struct mmu_notifier mmu_notifier;
> +#endif
>  	unsigned long mmu_invalidate_seq;
>  	long mmu_invalidate_in_progress;
>  	gfn_t mmu_invalidate_range_start;
>  	gfn_t mmu_invalidate_range_end;
> -#endif

Blech.  The existing code is a bit ugly, and trying to extend for this use case
makes things even worse.

Rather than use the base MMU_NOTIFIER Kconfig and an arbitrary define, I think we
should first add a proper Kconfig, e.g. KVM_GENERIC_MMU_NOTIFIER, to replace the
combination.  E.g

	config KVM_GENERIC_MMU_NOTIFIER
	       select MMU_NOTIFIER
	       bool

and then all architectures that currently #define KVM_ARCH_WANT_MMU_NOTIFIER can
simply select the Kconfig, which is everything except s390.  "GENERIC" again because
s390 does select MMU_NOTIFER and actually registers its own notifier for s390's
version of protected VMs (at least, I think that's what its "pv" stands for).

And then later down the line in this series, when the attributes and private mem
needs to tie into the notifiers, we can do:


	config KVM_GENERIC_MEMORY_ATTRIBUTES
	       select KVM_GENERIC_MMU_NOTIFIER
	       bool

I.e. that way this patch doesn't need to partially expose KVM's notifier stuff
and can instead just keep the soon-to-be-existing KVM_GENERIC_MMU_NOTIFIER.

Taking a depending on KVM_GENERIC_MMU_NOTIFIER for KVM_GENERIC_MEMORY_ATTRIBUTES
makes sense, because AFAICT, changing any type of attribute, e.g. RWX bits, is
going to necessitate unmapping the affected gfn range.

>  	struct list_head devices;
>  	u64 manual_dirty_log_protect;
>  	struct dentry *debugfs_dentry;
> @@ -1480,6 +1482,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
>  int kvm_arch_post_init_vm(struct kvm *kvm);
>  void kvm_arch_pre_destroy_vm(struct kvm *kvm);
>  int kvm_arch_create_vm_debugfs(struct kvm *kvm);
> +bool kvm_arch_has_private_mem(struct kvm *kvm);

The reference to private memory belongs in a later patch.  More below.

> +static void kvm_unmap_mem_range(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> +	struct kvm_gfn_range gfn_range;
> +	struct kvm_memory_slot *slot;
> +	struct kvm_memslots *slots;
> +	struct kvm_memslot_iter iter;
> +	int i;
> +	int r = 0;

The return from kvm_unmap_gfn_range() is a bool, this should be:

	bool flush = false;

> +
> +	gfn_range.pte = __pte(0);
> +	gfn_range.may_block = true;
> +
> +	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +		slots = __kvm_memslots(kvm, i);
> +
> +		kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> +			slot = iter.slot;
> +			gfn_range.start = max(start, slot->base_gfn);
> +			gfn_range.end = min(end, slot->base_gfn + slot->npages);
> +			if (gfn_range.start >= gfn_range.end)
> +				continue;
> +			gfn_range.slot = slot;
> +
> +			r |= kvm_unmap_gfn_range(kvm, &gfn_range);
> +		}
> +	}
> +
> +	if (r)
> +		kvm_flush_remote_tlbs(kvm);
> +}
> +
>  static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>  					   struct kvm_memory_attributes *attrs)
>  {
>  	gfn_t start, end;
>  	unsigned long i;
>  	void *entry;
> +	int idx;
>  	u64 supported_attrs = kvm_supported_mem_attributes(kvm);
>  
> -	/* flags is currently not used. */
> +	/* 'flags' is currently not used. */

Kind of a spurious change.

>  	if (attrs->flags)
>  		return -EINVAL;
>  	if (attrs->attributes & ~supported_attrs)
> @@ -2372,6 +2409,13 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>  
>  	entry = attrs->attributes ? xa_mk_value(attrs->attributes) : NULL;
>  
> +	if (kvm_arch_has_private_mem(kvm)) {

I think we should assume that any future attributes will necessitate unmapping
and invalidation, i.e. drop the private mem check.  That allows introducing
kvm_arch_has_private_mem() in a later patch that is more directly related to
private memory.

> +		KVM_MMU_LOCK(kvm);
> +		kvm_mmu_invalidate_begin(kvm);
> +		kvm_mmu_invalidate_range_add(kvm, start, end);
> +		KVM_MMU_UNLOCK(kvm);
> +	}
> +
>  	mutex_lock(&kvm->lock);
>  	for (i = start; i < end; i++)
>  		if (xa_err(xa_store(&kvm->mem_attr_array, i, entry,
> @@ -2379,6 +2423,16 @@ static int kvm_vm_ioctl_set_mem_attributes(struct kvm *kvm,
>  			break;
>  	mutex_unlock(&kvm->lock);
>  
> +	if (kvm_arch_has_private_mem(kvm)) {
> +		idx = srcu_read_lock(&kvm->srcu);

Mostly for reference, this goes away if slots_lock is used instead of kvm->lock.

> +		KVM_MMU_LOCK(kvm);
> +		if (i > start)
> +			kvm_unmap_mem_range(kvm, start, i);
> +		kvm_mmu_invalidate_end(kvm);
> +		KVM_MMU_UNLOCK(kvm);
> +		srcu_read_unlock(&kvm->srcu, idx);
> +	}
> +
>  	attrs->address = i << PAGE_SHIFT;
>  	attrs->size = (end - i) << PAGE_SHIFT;
>  
> -- 
> 2.25.1
>