Re: [Qemu-devel] [PATCH v2 3/4] vfio: Inhibit ballooning based on group attachment to a container

[Peter Xu <peterx@redhat.com>]

On Mon, Jul 30, 2018 at 05:14:21PM -0600, Alex Williamson wrote:
> We use a VFIOContainer to associate an AddressSpace to one or more
> VFIOGroups.  The VFIOContainer represents the DMA context for that
> AdressSpace for those VFIOGroups and is synchronized to changes in
> that AddressSpace via a MemoryListener.  For IOMMU backed devices,
> maintaining the DMA context for a VFIOGroup generally involves
> pinning a host virtual address in order to create a stable host
> physical address and then mapping a translation from the associated
> guest physical address to that host physical address into the IOMMU.
> 
> While the above maintains the VFIOContainer synchronized to the QEMU
> memory API of the VM, memory ballooning occurs outside of that API.
> Inflating the memory balloon (ie. cooperatively capturing pages from
> the guest for use by the host) simply uses MADV_DONTNEED to "zap"
> pages from QEMU's host virtual address space.  The page pinning and
> IOMMU mapping above remains in place, negating the host's ability to
> reuse the page, but the host virtual to host physical mapping of the
> page is invalidated outside of QEMU's memory API.
> 
> When the balloon is later deflated, attempting to cooperatively
> return pages to the guest, the page is simply freed by the guest
> balloon driver, allowing it to be used in the guest and incurring a
> page fault when that occurs.  The page fault maps a new host physical
> page backing the existing host virtual address, meanwhile the
> VFIOContainer still maintains the translation to the original host
> physical address.  At this point the guest vCPU and any assigned
> devices will map different host physical addresses to the same guest
> physical address.  Badness.
> 
> The IOMMU typically does not have page level granularity with which
> it can track this mapping without also incurring inefficiencies in
> using page size mappings throughout.  MMU notifiers in the host
> kernel also provide indicators for invalidating the mapping on
> balloon inflation, not for updating the mapping when the balloon is
> deflated.  For these reasons we assume a default behavior that the
> mapping of each VFIOGroup into the VFIOContainer is incompatible
> with memory ballooning and increment the balloon inhibitor to match
> the attached VFIOGroups.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>  hw/vfio/common.c |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index fb396cf00ac4..4881b691a659 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -32,6 +32,7 @@
>  #include "hw/hw.h"
>  #include "qemu/error-report.h"
>  #include "qemu/range.h"
> +#include "sysemu/balloon.h"
>  #include "sysemu/kvm.h"
>  #include "trace.h"
>  #include "qapi/error.h"
> @@ -1049,6 +1050,7 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as,
>              group->container = container;
>              QLIST_INSERT_HEAD(&container->group_list, group, container_next);
>              vfio_kvm_device_add_group(group);
> +            qemu_balloon_inhibit(true);

[1]

>              return 0;
>          }
>      }
> @@ -1198,6 +1200,7 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as,
>      }
>  
>      vfio_kvm_device_add_group(group);
> +    qemu_balloon_inhibit(true);

AFAIU there is a very critical information that this
qemu_balloon_inhibit() call must be before the call to:

    memory_listener_register(&container->listener, container->space->as);

Since the memory listener registeration is the point when we do the
pinning of the pages.  So to make sure we won't have stale pages we
must call qemu_balloon_inhibit() before memory_listener_register()
(which is what this patch does).  However this is not that obvious,
not sure whether that might worth a comment.

Considering this, not sure whether we can just do this per-container
instead of per-group, then we also don't need to bother with extra
group-add paths like [1].

No matter what, this patch looks good to me (and it is correct AFAIK),
so I'm leaving r-b and I'll leave Alex to decide:

Reviewed-by: Peter Xu <peterx@redhat.com>

>  
>      QLIST_INIT(&container->group_list);
>      QLIST_INSERT_HEAD(&space->containers, container, next);
> @@ -1222,6 +1225,7 @@ static int vfio_connect_container(VFIOGroup *group, AddressSpace *as,
>  listener_release_exit:
>      QLIST_REMOVE(group, container_next);
>      QLIST_REMOVE(container, next);
> +    qemu_balloon_inhibit(false);
>      vfio_kvm_device_del_group(group);
>      vfio_listener_release(container);
>  
> @@ -1352,6 +1356,7 @@ void vfio_put_group(VFIOGroup *group)
>          return;
>      }
>  
> +    qemu_balloon_inhibit(false);
>      vfio_kvm_device_del_group(group);
>      vfio_disconnect_container(group);
>      QLIST_REMOVE(group, next);
> 

Regards,

-- 
Peter Xu