Re: [PATCH v5 05/14] vfio/migration: Allow migration without VFIO IOMMU dirty tracking support

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 29 Dec 2022 13:03:36 +0200
Avihai Horon <avihaih@nvidia.com> wrote:

> Currently, if IOMMU of a VFIO container doesn't support dirty page
> tracking, migration is blocked. This is because a DMA-able VFIO device
> can dirty RAM pages without updating QEMU about it, thus breaking the
> migration.
> 
> However, this doesn't mean that migration can't be done at all.
> In such case, allow migration and let QEMU VFIO code mark the entire
> bitmap dirty.
> 
> This guarantees that all pages that might have gotten dirty are reported
> back, and thus guarantees a valid migration even without VFIO IOMMU
> dirty tracking support.
> 
> The motivation for this patch is the introduction of iommufd [1].
> iommufd can directly implement the /dev/vfio/vfio container IOCTLs by
> mapping them into its internal ops, allowing the usage of these IOCTLs
> over iommufd. However, VFIO IOMMU dirty tracking is not supported by
> this VFIO compatibility API.
> 
> This patch will allow migration by hosts that use the VFIO compatibility
> API and prevent migration regressions caused by the lack of VFIO IOMMU
> dirty tracking support.
> 
> [1]
> https://lore.kernel.org/kvm/0-v6-a196d26f289e+11787-iommufd_jgg@nvidia.com/
> 
> Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> ---
>  hw/vfio/common.c    | 100 ++++++++++++++++++++++++++------------------
>  hw/vfio/migration.c |   3 +-
>  2 files changed, 61 insertions(+), 42 deletions(-)
> 
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index 130e5d1dc7..67104e2fc2 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -397,51 +397,61 @@ static int vfio_dma_unmap_bitmap(VFIOContainer *container,
>                                   IOMMUTLBEntry *iotlb)
>  {
>      struct vfio_iommu_type1_dma_unmap *unmap;
> -    struct vfio_bitmap *bitmap;
> +    struct vfio_bitmap *vbitmap;
> +    unsigned long *bitmap;
> +    uint64_t bitmap_size;
>      uint64_t pages = REAL_HOST_PAGE_ALIGN(size) / qemu_real_host_page_size();
>      int ret;
>  
> -    unmap = g_malloc0(sizeof(*unmap) + sizeof(*bitmap));
> +    unmap = g_malloc0(sizeof(*unmap) + sizeof(*vbitmap));
>  
> -    unmap->argsz = sizeof(*unmap) + sizeof(*bitmap);
> +    unmap->argsz = sizeof(*unmap);
>      unmap->iova = iova;
>      unmap->size = size;
> -    unmap->flags |= VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP;
> -    bitmap = (struct vfio_bitmap *)&unmap->data;
>  
> +    bitmap_size = ROUND_UP(pages, sizeof(__u64) * BITS_PER_BYTE) /
> +                  BITS_PER_BYTE;
> +    bitmap = g_try_malloc0(bitmap_size);
> +    if (!bitmap) {
> +        ret = -ENOMEM;
> +        goto unmap_exit;
> +    }
> +
> +    if (!container->dirty_pages_supported) {
> +        bitmap_set(bitmap, 0, pages);
> +        goto do_unmap;
> +    }
> +
> +    unmap->argsz += sizeof(*vbitmap);
> +    unmap->flags = VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP;
> +
> +    vbitmap = (struct vfio_bitmap *)&unmap->data;
> +    vbitmap->data = (__u64 *)bitmap;
>      /*
>       * cpu_physical_memory_set_dirty_lebitmap() supports pages in bitmap of
>       * qemu_real_host_page_size to mark those dirty. Hence set bitmap_pgsize
>       * to qemu_real_host_page_size.
>       */
> +    vbitmap->pgsize = qemu_real_host_page_size();
> +    vbitmap->size = bitmap_size;
>  
> -    bitmap->pgsize = qemu_real_host_page_size();
> -    bitmap->size = ROUND_UP(pages, sizeof(__u64) * BITS_PER_BYTE) /
> -                   BITS_PER_BYTE;
> -
> -    if (bitmap->size > container->max_dirty_bitmap_size) {
> -        error_report("UNMAP: Size of bitmap too big 0x%"PRIx64,
> -                     (uint64_t)bitmap->size);
> +    if (bitmap_size > container->max_dirty_bitmap_size) {
> +        error_report("UNMAP: Size of bitmap too big 0x%"PRIx64, bitmap_size);
>          ret = -E2BIG;
>          goto unmap_exit;
>      }
>  
> -    bitmap->data = g_try_malloc0(bitmap->size);
> -    if (!bitmap->data) {
> -        ret = -ENOMEM;
> -        goto unmap_exit;
> -    }
> -
> +do_unmap:
>      ret = ioctl(container->fd, VFIO_IOMMU_UNMAP_DMA, unmap);
>      if (!ret) {
> -        cpu_physical_memory_set_dirty_lebitmap((unsigned long *)bitmap->data,
> -                iotlb->translated_addr, pages);
> +        cpu_physical_memory_set_dirty_lebitmap(bitmap, iotlb->translated_addr,
> +                                               pages);
>      } else {
>          error_report("VFIO_UNMAP_DMA with DIRTY_BITMAP : %m");
>      }
>  
> -    g_free(bitmap->data);
>  unmap_exit:
> +    g_free(bitmap);
>      g_free(unmap);
>      return ret;
>  }
> @@ -460,8 +470,7 @@ static int vfio_dma_unmap(VFIOContainer *container,
>          .size = size,
>      };
>  
> -    if (iotlb && container->dirty_pages_supported &&
> -        vfio_devices_all_running_and_saving(container)) {
> +    if (iotlb && vfio_devices_all_running_and_saving(container)) {
>          return vfio_dma_unmap_bitmap(container, iova, size, iotlb);
>      }

Seems like it would be simpler to follow the non-dirty_pages_supported
path here and follow-up with a condition to call
cpu_physical_memory_set_dirty_range() rather than try to make the
bitmap path serve double-duty.

>  
> @@ -1201,6 +1210,10 @@ static void vfio_set_dirty_page_tracking(VFIOContainer *container, bool start)
>          .argsz = sizeof(dirty),
>      };
>  
> +    if (!container->dirty_pages_supported) {
> +        return;
> +    }
> +
>      if (start) {
>          dirty.flags = VFIO_IOMMU_DIRTY_PAGES_FLAG_START;
>      } else {
> @@ -1231,11 +1244,26 @@ static void vfio_listener_log_global_stop(MemoryListener *listener)
>  static int vfio_get_dirty_bitmap(VFIOContainer *container, uint64_t iova,
>                                   uint64_t size, ram_addr_t ram_addr)
>  {
> -    struct vfio_iommu_type1_dirty_bitmap *dbitmap;
> +    struct vfio_iommu_type1_dirty_bitmap *dbitmap = NULL;
>      struct vfio_iommu_type1_dirty_bitmap_get *range;
> +    unsigned long *bitmap;
> +    uint64_t bitmap_size;
>      uint64_t pages;
>      int ret;
>  
> +    pages = REAL_HOST_PAGE_ALIGN(size) / qemu_real_host_page_size();
> +    bitmap_size = ROUND_UP(pages, sizeof(__u64) * BITS_PER_BYTE) /
> +                                  BITS_PER_BYTE;
> +    bitmap = g_try_malloc0(bitmap_size);
> +    if (!bitmap) {
> +        return -ENOMEM;
> +    }
> +
> +    if (!container->dirty_pages_supported) {
> +        bitmap_set(bitmap, 0, pages);
> +        goto set_dirty;
> +    }

Same here, why are we bothering to allocate and mark a bitmap dirty
just to appease code we don't need instead of:

	if (!container->dirty_pages_supported) {
		cpu_physical_memory_set_dirty_range(...)
		return 0;
	}

Thanks,
Alex

> +
>      dbitmap = g_malloc0(sizeof(*dbitmap) + sizeof(*range));
>  
>      dbitmap->argsz = sizeof(*dbitmap) + sizeof(*range);
> @@ -1250,15 +1278,8 @@ static int vfio_get_dirty_bitmap(VFIOContainer *container, uint64_t iova,
>       * to qemu_real_host_page_size.
>       */
>      range->bitmap.pgsize = qemu_real_host_page_size();
> -
> -    pages = REAL_HOST_PAGE_ALIGN(range->size) / qemu_real_host_page_size();
> -    range->bitmap.size = ROUND_UP(pages, sizeof(__u64) * BITS_PER_BYTE) /
> -                                         BITS_PER_BYTE;
> -    range->bitmap.data = g_try_malloc0(range->bitmap.size);
> -    if (!range->bitmap.data) {
> -        ret = -ENOMEM;
> -        goto err_out;
> -    }
> +    range->bitmap.size = bitmap_size;
> +    range->bitmap.data = (__u64 *)bitmap;
>  
>      ret = ioctl(container->fd, VFIO_IOMMU_DIRTY_PAGES, dbitmap);
>      if (ret) {
> @@ -1268,13 +1289,13 @@ static int vfio_get_dirty_bitmap(VFIOContainer *container, uint64_t iova,
>          goto err_out;
>      }
>  
> -    cpu_physical_memory_set_dirty_lebitmap((unsigned long *)range->bitmap.data,
> -                                            ram_addr, pages);
> +set_dirty:
> +    cpu_physical_memory_set_dirty_lebitmap(bitmap, ram_addr, pages);
>  
> -    trace_vfio_get_dirty_bitmap(container->fd, range->iova, range->size,
> -                                range->bitmap.size, ram_addr);
> +    trace_vfio_get_dirty_bitmap(container->fd, iova, size, bitmap_size,
> +                                ram_addr);
>  err_out:
> -    g_free(range->bitmap.data);
> +    g_free(bitmap);
>      g_free(dbitmap);
>  
>      return ret;
> @@ -1409,8 +1430,7 @@ static void vfio_listener_log_sync(MemoryListener *listener,
>  {
>      VFIOContainer *container = container_of(listener, VFIOContainer, listener);
>  
> -    if (vfio_listener_skipped_section(section) ||
> -        !container->dirty_pages_supported) {
> +    if (vfio_listener_skipped_section(section)) {
>          return;
>      }
>  
> diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c
> index 09fe7c1de2..552c2313b2 100644
> --- a/hw/vfio/migration.c
> +++ b/hw/vfio/migration.c
> @@ -860,11 +860,10 @@ int64_t vfio_mig_bytes_transferred(void)
>  
>  int vfio_migration_probe(VFIODevice *vbasedev, Error **errp)
>  {
> -    VFIOContainer *container = vbasedev->group->container;
>      struct vfio_region_info *info = NULL;
>      int ret = -ENOTSUP;
>  
> -    if (!vbasedev->enable_migration || !container->dirty_pages_supported) {
> +    if (!vbasedev->enable_migration) {
>          goto add_blocker;
>      }
>