Re: [PATCH v2 02/42] KVM: s390/interrupt: do not pin adapter interrupt pages

[David Hildenbrand <david@redhat.com>]

On 14.02.20 23:26, Christian Borntraeger wrote:
> From: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
> 
> The adapter interrupt page containing the indicator bits is currently
> pinned. That means that a guest with many devices can pin a lot of
> memory pages in the host. This also complicates the reference tracking
> which is needed for memory management handling of protected virtual
> machines. It might also have some strange side effects for madvise
> MADV_DONTNEED and other things.
> 
> We can simply try to get the userspace page set the bits and free the
> page. By storing the userspace address in the irq routing entry instead
> of the guest address we can actually avoid many lookups and list walks
> so that this variant is very likely not slower.
> 
> If userspace messes around with the memory slots the worst thing that
> can happen is that we write to some other memory within that process.
> As we get the the page with FOLL_WRITE this can also not be used to
> write to shared read-only pages.
> 
> Signed-off-by: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
> [borntraeger@de.ibm.com: patch simplification]
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> ---
>  Documentation/virt/kvm/devices/s390_flic.rst |  11 +-
>  arch/s390/include/asm/kvm_host.h             |   3 -
>  arch/s390/kvm/interrupt.c                    | 170 ++++++-------------
>  3 files changed, 53 insertions(+), 131 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/devices/s390_flic.rst b/Documentation/virt/kvm/devices/s390_flic.rst
> index 954190da7d04..ea96559ba501 100644
> --- a/Documentation/virt/kvm/devices/s390_flic.rst
> +++ b/Documentation/virt/kvm/devices/s390_flic.rst
> @@ -108,16 +108,9 @@ Groups:
>        mask or unmask the adapter, as specified in mask
>  
>      KVM_S390_IO_ADAPTER_MAP
> -      perform a gmap translation for the guest address provided in addr,
> -      pin a userspace page for the translated address and add it to the
> -      list of mappings
> -
> -      .. note:: A new mapping will be created unconditionally; therefore,
> -	        the calling code should avoid making duplicate mappings.
> -
> +      This is now a no-op. The mapping is purely done by the irq route.
>      KVM_S390_IO_ADAPTER_UNMAP
> -      release a userspace page for the translated address specified in addr
> -      from the list of mappings
> +      This is now a no-op. The mapping is purely done by the irq route.
>  

The interface should have accepted a hva from the very start and not
guest addresses ...

[...]

>  
>  static int modify_io_adapter(struct kvm_device *dev,
> @@ -2456,12 +2378,13 @@ static int modify_io_adapter(struct kvm_device *dev,
>  		if (ret > 0)
>  			ret = 0;
>  		break;
> +	/*
> +	 * We resolve the gpa to hva when setting the IRQ routing. the set_irq
> +	 * code uses get_user_pages_remote to do the actual write.

nit: "get_user_pages_remote()"

> +	 */
>  	case KVM_S390_IO_ADAPTER_MAP:
> -		ret = kvm_s390_adapter_map(dev->kvm, req.id, req.addr);
> -		break;
>  	case KVM_S390_IO_ADAPTER_UNMAP:
> -		ret = kvm_s390_adapter_unmap(dev->kvm, req.id, req.addr);
> -		break;
> +		return 0;
>  	default:
>  		ret = -EINVAL;
>  	}
> @@ -2699,19 +2622,21 @@ static unsigned long get_ind_bit(__u64 addr, unsigned long bit_nr, bool swap)
>  	return swap ? (bit ^ (BITS_PER_LONG - 1)) : bit;
>  }
>  
> -static struct s390_map_info *get_map_info(struct s390_io_adapter *adapter,
> -					  u64 addr)
> +static struct page *get_map_page(struct kvm *kvm,
> +				 struct s390_io_adapter *adapter,
> +				 u64 uaddr)
>  {
> -	struct s390_map_info *map;
> +	struct page *page = NULL;
>  
>  	if (!adapter)
>  		return NULL;

AFAIKs, this check is not necessary.

> -
> -	list_for_each_entry(map, &adapter->maps, list) {
> -		if (map->guest_addr == addr)
> -			return map;
> -	}
> -	return NULL;
> +	if (!uaddr)
> +		return NULL;

I do wonder if that check is necessary. I don't think so but might be
missing something.

> +	down_read(&kvm->mm->mmap_sem);
> +	get_user_pages_remote(NULL, kvm->mm, uaddr, 1, FOLL_WRITE,
> +			      &page, NULL, NULL);
> +	up_read(&kvm->mm->mmap_sem);
> +	return page;
>  }
>  
>  static int adapter_indicators_set(struct kvm *kvm,
> @@ -2720,30 +2645,35 @@ static int adapter_indicators_set(struct kvm *kvm,
>  {
>  	unsigned long bit;
>  	int summary_set, idx;
> -	struct s390_map_info *info;
> +	struct page *ind_page, *summary_page;
>  	void *map;
>  
> -	info = get_map_info(adapter, adapter_int->ind_addr);
> -	if (!info)
> +	ind_page = get_map_page(kvm, adapter, adapter_int->ind_addr);
> +	if (!ind_page)
>  		return -1;
> -	map = page_address(info->page);
> -	bit = get_ind_bit(info->addr, adapter_int->ind_offset, adapter->swap);
> -	set_bit(bit, map);
> -	idx = srcu_read_lock(&kvm->srcu);
> -	mark_page_dirty(kvm, info->guest_addr >> PAGE_SHIFT);
> -	set_page_dirty_lock(info->page);
> -	info = get_map_info(adapter, adapter_int->summary_addr);
> -	if (!info) {
> -		srcu_read_unlock(&kvm->srcu, idx);
> +	summary_page = get_map_page(kvm, adapter, adapter_int->summary_addr);
> +	if (!summary_page) {
> +		put_page(ind_page);
>  		return -1;
>  	}
> -	map = page_address(info->page);
> -	bit = get_ind_bit(info->addr, adapter_int->summary_offset,
> -			  adapter->swap);
> +
> +	idx = srcu_read_lock(&kvm->srcu);
> +	map = page_address(ind_page);
> +	bit = get_ind_bit(adapter_int->ind_addr,
> +			  adapter_int->ind_offset, adapter->swap);
> +	set_bit(bit, map);
> +	mark_page_dirty(kvm, adapter_int->ind_addr >> PAGE_SHIFT);
> +	set_page_dirty_lock(ind_page);
> +	map = page_address(summary_page);
> +	bit = get_ind_bit(adapter_int->summary_addr,
> +			  adapter_int->summary_offset, adapter->swap);
>  	summary_set = test_and_set_bit(bit, map);
> -	mark_page_dirty(kvm, info->guest_addr >> PAGE_SHIFT);
> -	set_page_dirty_lock(info->page);
> +	mark_page_dirty(kvm, adapter_int->summary_addr >> PAGE_SHIFT);
> +	set_page_dirty_lock(summary_page);
>  	srcu_read_unlock(&kvm->srcu, idx);
> +
> +	put_page(ind_page);
> +	put_page(summary_page);
>  	return summary_set ? 0 : 1;
>  }
>  
> @@ -2765,9 +2695,7 @@ static int set_adapter_int(struct kvm_kernel_irq_routing_entry *e,
>  	adapter = get_io_adapter(kvm, e->adapter.adapter_id);
>  	if (!adapter)
>  		return -1;
> -	down_read(&adapter->maps_lock);
>  	ret = adapter_indicators_set(kvm, adapter, &e->adapter);
> -	up_read(&adapter->maps_lock);
>  	if ((ret > 0) && !adapter->masked) {
>  		ret = kvm_s390_inject_airq(kvm, adapter);
>  		if (ret == 0)
> @@ -2818,23 +2746,27 @@ int kvm_set_routing_entry(struct kvm *kvm,
>  			  struct kvm_kernel_irq_routing_entry *e,
>  			  const struct kvm_irq_routing_entry *ue)
>  {
> -	int ret;
> +	u64 uaddr;
>  
>  	switch (ue->type) {
> +	/* we store the userspace addresses instead of the guest addresses */
>  	case KVM_IRQ_ROUTING_S390_ADAPTER:
>  		e->set = set_adapter_int;
> -		e->adapter.summary_addr = ue->u.adapter.summary_addr;
> -		e->adapter.ind_addr = ue->u.adapter.ind_addr;
> +		uaddr =  gmap_translate(kvm->arch.gmap, ue->u.adapter.summary_addr);
> +		if (uaddr == -EFAULT)
> +			return -EFAULT;
> +		e->adapter.summary_addr = uaddr;
> +		uaddr =  gmap_translate(kvm->arch.gmap, ue->u.adapter.ind_addr);
> +		if (uaddr == -EFAULT)
> +			return -EFAULT;

AFAIK, leaving e->adapter.summary_addr set is not an issue.

Interesting, in kvm_s390_adapter_map(), we didn't synchronize again slot
updates when doing the gmap_translate(), which looks wrong to me ...

It seems to be the same thing here. I do wonder if it is safe to do a
gmap_translate() here, looks like this can race with
kvm_arch_commit_memory_region().

I would have assumed we need e.g., the slots_lock while doing the
gmap_translate() - or a srcu_read_lock(&vcpu->kvm->srcu) or similar ...


Apart from that, looks good to me.

-- 
Thanks,

David / dhildenb