Re: [Qemu-devel] [PATCH v2] migration: discard non-migratable RAMBlocks

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Cédric Le Goater (clg@kaod.org) wrote:
> On the POWER9 processor, the XIVE interrupt controller can control
> interrupt sources using MMIO to trigger events, to EOI or to turn off
> the sources. Priority management and interrupt acknowledgment is also
> controlled by MMIO in the presenter sub-engine.
> 
> These MMIO regions are exposed to guests in QEMU with a set of 'ram
> device' memory mappings, similarly to VFIO, and the VMAs are populated
> dynamically with the appropriate pages using a fault handler.
> 
> But, these regions are an issue for migration. We need to discard the
> associated RAMBlocks from the RAM state on the source VM and let the
> destination VM rebuild the memory mappings on the new host in the
> post_load() operation just before resuming the system.
> 
> To achieve this goal, the following introduces a new RAMBlock flag
> RAM_MIGRATABLE which is updated in the vmstate_register_ram() and
> vmstate_unregister_ram() routines. This flag is then used by the
> migration to identify RAMBlocks to discard on the source. Some checks
> are also performed on the destination to make sure nothing invalid was
> sent.
> 
> Signed-off-by: Cédric Le Goater <clg@kaod.org>

Hi Cédric,
  Yes, this is looking nicer; the macro makes the changes quite small.
A couple of questions:

> ---
> 
>  Changes since v1:
> 
>  - introduced a new RAMBlock flag RAM_MIGRATABLE
>  - fixed RAMBLOCK_FOREACH_MIGRATABLE() macro to handle code that
>    omitted {}. 
> 
>  exec.c                    | 10 ++++++++++
>  include/exec/cpu-common.h |  1 +
>  migration/ram.c           | 42 ++++++++++++++++++++++++++++++++----------
>  3 files changed, 43 insertions(+), 10 deletions(-)
> 
> diff --git a/exec.c b/exec.c
> index 02b1efebb7c3..e9432c06294e 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -104,6 +104,9 @@ static MemoryRegion io_mem_unassigned;
>   * (Set during postcopy)
>   */
>  #define RAM_UF_ZEROPAGE (1 << 3)
> +
> +/* RAM can be migrated */
> +#define RAM_MIGRATABLE (1 << 4)
>  #endif
>  
>  #ifdef TARGET_PAGE_BITS_VARY
> @@ -1807,6 +1810,11 @@ void qemu_ram_set_uf_zeroable(RAMBlock *rb)
>      rb->flags |= RAM_UF_ZEROPAGE;
>  }
>  
> +bool qemu_ram_is_migratable(RAMBlock *rb)
> +{
> +    return rb->flags & RAM_MIGRATABLE;
> +}
> +
>  /* Called with iothread lock held.  */
>  void qemu_ram_set_idstr(RAMBlock *new_block, const char *name, DeviceState *dev)
>  {
> @@ -1823,6 +1831,7 @@ void qemu_ram_set_idstr(RAMBlock *new_block, const char *name, DeviceState *dev)
>          }
>      }
>      pstrcat(new_block->idstr, sizeof(new_block->idstr), name);
> +    new_block->flags |= RAM_MIGRATABLE;
>  
>      rcu_read_lock();
>      RAMBLOCK_FOREACH(block) {
> @@ -1845,6 +1854,7 @@ void qemu_ram_unset_idstr(RAMBlock *block)
>       */
>      if (block) {
>          memset(block->idstr, 0, sizeof(block->idstr));
> +        block->flags &= ~RAM_MIGRATABLE;
>      }
>  }

Why in qemu_ram_set_idstr and qemu_ram_(un)set_idstr ?  It seems an
odd place to put them.


> diff --git a/include/exec/cpu-common.h b/include/exec/cpu-common.h
> index 24d335f95d45..96854519b514 100644
> --- a/include/exec/cpu-common.h
> +++ b/include/exec/cpu-common.h
> @@ -75,6 +75,7 @@ const char *qemu_ram_get_idstr(RAMBlock *rb);
>  bool qemu_ram_is_shared(RAMBlock *rb);
>  bool qemu_ram_is_uf_zeroable(RAMBlock *rb);
>  void qemu_ram_set_uf_zeroable(RAMBlock *rb);
> +bool qemu_ram_is_migratable(RAMBlock *rb);
>  
>  size_t qemu_ram_pagesize(RAMBlock *block);
>  size_t qemu_ram_pagesize_largest(void);
> diff --git a/migration/ram.c b/migration/ram.c
> index 0e90efa09236..89c3accc4e26 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -187,6 +187,11 @@ void ramblock_recv_bitmap_set_range(RAMBlock *rb, void *host_addr,
>                        nr);
>  }
>  
> +/* Should be holding either ram_list.mutex, or the RCU lock. */
> +#define RAMBLOCK_FOREACH_MIGRATABLE(block)             \
> +    RAMBLOCK_FOREACH(block)                            \
> +        if (!qemu_ram_is_migratable(block)) {} else
> +
>  /*
>   * An outstanding page request, on the source, having been received
>   * and queued
> @@ -780,6 +785,10 @@ unsigned long migration_bitmap_find_dirty(RAMState *rs, RAMBlock *rb,
>      unsigned long *bitmap = rb->bmap;
>      unsigned long next;
>  
> +    if (!qemu_ram_is_migratable(rb)) {
> +        return size;
> +    }
> +
>      if (rs->ram_bulk_stage && start > 0) {
>          next = start + 1;
>      } else {
> @@ -825,7 +834,7 @@ uint64_t ram_pagesize_summary(void)
>      RAMBlock *block;
>      uint64_t summary = 0;
>  
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          summary |= block->page_size;
>      }
>  
> @@ -849,7 +858,7 @@ static void migration_bitmap_sync(RAMState *rs)
>  
>      qemu_mutex_lock(&rs->bitmap_mutex);
>      rcu_read_lock();
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          migration_bitmap_sync_range(rs, block, 0, block->used_length);
>      }
>      rcu_read_unlock();
> @@ -1499,6 +1508,10 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss,
>      size_t pagesize_bits =
>          qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
>  
> +    if (!qemu_ram_is_migratable(pss->block)) {
> +        return 0;
> +    }
> +

We might want to print a diagnostic there - it shouldn't happen right?

>      do {
>          tmppages = ram_save_target_page(rs, pss, last_stage);
>          if (tmppages < 0) {
> @@ -1587,7 +1600,7 @@ uint64_t ram_bytes_total(void)
>      uint64_t total = 0;
>  
>      rcu_read_lock();
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          total += block->used_length;
>      }
>      rcu_read_unlock();
> @@ -1642,7 +1655,7 @@ static void ram_save_cleanup(void *opaque)
>       */
>      memory_global_dirty_log_stop();
>  
> -    QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          g_free(block->bmap);
>          block->bmap = NULL;
>          g_free(block->unsentmap);
> @@ -1705,7 +1718,7 @@ void ram_postcopy_migrated_memory_release(MigrationState *ms)
>  {
>      struct RAMBlock *block;
>  
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          unsigned long *bitmap = block->bmap;
>          unsigned long range = block->used_length >> TARGET_PAGE_BITS;
>          unsigned long run_start = find_next_zero_bit(bitmap, range, 0);
> @@ -1783,7 +1796,7 @@ static int postcopy_each_ram_send_discard(MigrationState *ms)
>      struct RAMBlock *block;
>      int ret;
>  
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          PostcopyDiscardState *pds =
>              postcopy_discard_send_init(ms, block->idstr);
>  
> @@ -1991,7 +2004,7 @@ int ram_postcopy_send_discard_bitmap(MigrationState *ms)
>      rs->last_sent_block = NULL;
>      rs->last_page = 0;
>  
> -    QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          unsigned long pages = block->used_length >> TARGET_PAGE_BITS;
>          unsigned long *bitmap = block->bmap;
>          unsigned long *unsentmap = block->unsentmap;
> @@ -2150,7 +2163,7 @@ static void ram_list_init_bitmaps(void)
>  
>      /* Skip setting bitmap if there is no RAM */
>      if (ram_bytes_total()) {
> -        QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
> +        RAMBLOCK_FOREACH_MIGRATABLE(block) {
>              pages = block->max_length >> TARGET_PAGE_BITS;
>              block->bmap = bitmap_new(pages);
>              bitmap_set(block->bmap, 0, pages);
> @@ -2226,7 +2239,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>  
>      qemu_put_be64(f, ram_bytes_total() | RAM_SAVE_FLAG_MEM_SIZE);
>  
> -    RAMBLOCK_FOREACH(block) {
> +    RAMBLOCK_FOREACH_MIGRATABLE(block) {
>          qemu_put_byte(f, strlen(block->idstr));
>          qemu_put_buffer(f, (uint8_t *)block->idstr, strlen(block->idstr));
>          qemu_put_be64(f, block->used_length);

Good, that's all quite neat;  we've just got to watch out we don't
accidentally add any RAMBLOCK_FOREACH's back.

> @@ -2471,6 +2484,11 @@ static inline RAMBlock *ram_block_from_stream(QEMUFile *f, int flags)
>          return NULL;
>      }
>  
> +    if (!qemu_ram_is_migratable(block)) {
> +        error_report("block %s should not be migrated !", id);
> +        return NULL;
> +    }
> +
>      return block;
>  }
>  
> @@ -2921,7 +2939,11 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>                  length = qemu_get_be64(f);
>  
>                  block = qemu_ram_block_by_name(id);
> -                if (block) {
> +                if (block && !qemu_ram_is_migratable(block)) {
> +                    error_report("block %s should not be migrated !", id);
> +                    ret = -EINVAL;
> +
> +                } else if (block) {
>                      if (length != block->used_length) {
>                          Error *local_err = NULL;
>  
> -- 
> 2.13.6

Dave

> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK