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

[Avihai Horon]

On 06/01/2023 23:56, Alex Williamson wrote:
> External email: Use caution opening links or attachments
>
>
> 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.

Nice, I wasn't aware of cpu_physical_memory_set_dirty_range().
It really simplifies things. I will change it.

> > @@ -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;
>          }

I will change here as well.

Thanks!

> 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;
> >       }