On 17.02.20 10:43, David Hildenbrand wrote:
> On 14.02.20 23:26, Christian Borntraeger wrote:
>> From: Ulrich Weigand <Ulrich.Weigand@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> [borntraeger@xxxxxxxxxx: patch simplification]
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> ---
>> 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 ...
right, but that is history.
>
> [...]
>> + /*
>> + * 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()"
ack.
>
>> + */
>> 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.
Right otherwise we would crash earlier.
>
>> -
>> - 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.
Nothing should break when we remove this check. get_user_pages_remote will
also return NULL (as newer kernels usually forbid mapping things at 0).
Will remove.
[...]
>> @@ -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 ...
gmap_translate does this via the gmap and it holds the mm sem. gmap_unmap_segment
takes the same lock. So I think we are ok here.
>
> Apart from that, looks good to me.
>
