Re: [Qemu-devel] [RFC PATCH] migration: discard RAMBlocks of type ram_device

[Kirti Wankhede]

On 4/11/2018 11:25 PM, Alex Williamson wrote:
> [cc +folks working on vfio-mdev migration]
> 
> On Wed, 11 Apr 2018 19:20:14 +0200
> Cédric Le Goater <address@hidden> wrote:
> 
>> Here is some context for this strange change request.
>>
>> 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 subengine.
>>
>> 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.
>>
>> This is the goal of the following proposal. Does it make sense ? It
>> seems to be working enough to migrate a running guest but there might
>> be a better, more subtle, approach.
> 
> Yulei, is this something you've run into with GVT-g migration?  I don't
> see how we can read from or write to ram_device regions in a useful way
> during migration anyway, so the change initially looks correct to me.
> Thanks,
> 

I ran into this problem with vGPU migration. I have very similar patch
in my local branch to test vGPU migration. This patch looks good to me.

Thanks,
Kirti


> Alex
> 
>> Signed-off-by: Cédric Le Goater <address@hidden>
>> ---
>>  migration/ram.c | 42 ++++++++++++++++++++++++++++++++++++++++--
>>  1 file changed, 40 insertions(+), 2 deletions(-)
>>
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 0e90efa09236..6404ccd046d8 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -780,6 +780,10 @@ unsigned long migration_bitmap_find_dirty(RAMState *rs, 
>> RAMBlock *rb,
>>      unsigned long *bitmap = rb->bmap;
>>      unsigned long next;
>>  
>> +    if (memory_region_is_ram_device(rb->mr)) {
>> +        return size;
>> +    }
>> +
>>      if (rs->ram_bulk_stage && start > 0) {
>>          next = start + 1;
>>      } else {
>> @@ -826,6 +830,9 @@ uint64_t ram_pagesize_summary(void)
>>      uint64_t summary = 0;
>>  
>>      RAMBLOCK_FOREACH(block) {
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          summary |= block->page_size;
>>      }
>>  
>> @@ -850,6 +857,9 @@ static void migration_bitmap_sync(RAMState *rs)
>>      qemu_mutex_lock(&rs->bitmap_mutex);
>>      rcu_read_lock();
>>      RAMBLOCK_FOREACH(block) {
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          migration_bitmap_sync_range(rs, block, 0, block->used_length);
>>      }
>>      rcu_read_unlock();
>> @@ -1499,6 +1509,10 @@ static int ram_save_host_page(RAMState *rs, 
>> PageSearchStatus *pss,
>>      size_t pagesize_bits =
>>          qemu_ram_pagesize(pss->block) >> TARGET_PAGE_BITS;
>>  
>> +    if (memory_region_is_ram_device(pss->block->mr)) {
>> +        return 0;
>> +    }
>> +
>>      do {
>>          tmppages = ram_save_target_page(rs, pss, last_stage);
>>          if (tmppages < 0) {
>> @@ -1588,6 +1602,9 @@ uint64_t ram_bytes_total(void)
>>  
>>      rcu_read_lock();
>>      RAMBLOCK_FOREACH(block) {
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          total += block->used_length;
>>      }
>>      rcu_read_unlock();
>> @@ -1643,6 +1660,9 @@ static void ram_save_cleanup(void *opaque)
>>      memory_global_dirty_log_stop();
>>  
>>      QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          g_free(block->bmap);
>>          block->bmap = NULL;
>>          g_free(block->unsentmap);
>> @@ -1710,6 +1730,9 @@ void 
>> ram_postcopy_migrated_memory_release(MigrationState *ms)
>>          unsigned long range = block->used_length >> TARGET_PAGE_BITS;
>>          unsigned long run_start = find_next_zero_bit(bitmap, range, 0);
>>  
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          while (run_start < range) {
>>              unsigned long run_end = find_next_bit(bitmap, range, run_start 
>> + 1);
>>              ram_discard_range(block->idstr, run_start << TARGET_PAGE_BITS,
>> @@ -1784,8 +1807,13 @@ static int 
>> postcopy_each_ram_send_discard(MigrationState *ms)
>>      int ret;
>>  
>>      RAMBLOCK_FOREACH(block) {
>> -        PostcopyDiscardState *pds =
>> -            postcopy_discard_send_init(ms, block->idstr);
>> +        PostcopyDiscardState *pds;
>> +
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>> +
>> +        pds = postcopy_discard_send_init(ms, block->idstr);
>>  
>>          /*
>>           * Postcopy sends chunks of bitmap over the wire, but it
>> @@ -1996,6 +2024,10 @@ int ram_postcopy_send_discard_bitmap(MigrationState 
>> *ms)
>>          unsigned long *bitmap = block->bmap;
>>          unsigned long *unsentmap = block->unsentmap;
>>  
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>> +
>>          if (!unsentmap) {
>>              /* We don't have a safe way to resize the sentmap, so
>>               * if the bitmap was resized it will be NULL at this
>> @@ -2151,6 +2183,9 @@ 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) {
>> +            if (memory_region_is_ram_device(block->mr)) {
>> +                continue;
>> +            }
>>              pages = block->max_length >> TARGET_PAGE_BITS;
>>              block->bmap = bitmap_new(pages);
>>              bitmap_set(block->bmap, 0, pages);
>> @@ -2227,6 +2262,9 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>>      qemu_put_be64(f, ram_bytes_total() | RAM_SAVE_FLAG_MEM_SIZE);
>>  
>>      RAMBLOCK_FOREACH(block) {
>> +        if (memory_region_is_ram_device(block->mr)) {
>> +            continue;
>> +        }
>>          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);
>