Re: [Qemu-devel] [PATCH] exec: fix access to ram_list.dirty_memory when sync dirty bitmap

[Paolo Bonzini]

On 28/06/2017 09:30, Juan Quintela wrote:
> Haozhong Zhang <address@hidden> wrote:
>> In cpu_physical_memory_sync_dirty_bitmap(rb, start, ...), the 2nd
>> argument 'start' is relative to the start of the ramblock 'rb'. When
>> it's used to access the dirty memory bitmap of ram_list (i.e.
>> ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]->blocks[]), an offset to
>> the start of all RAM (i.e. rb->offset) should be added to it, which has
>> however been missed since c/s 6b6712efcc. For a ramblock of host memory
>> backend whose offset is not zero, cpu_physical_memory_sync_dirty_bitmap()
>> synchronizes the incorrect part of the dirty memory bitmap of ram_list
>> to the per ramblock dirty bitmap. As a result, a guest with host
>> memory backend may crash after migration.
>>
>> Fix it by adding the offset of ramblock when accessing the dirty memory
>> bitmap of ram_list in cpu_physical_memory_sync_dirty_bitmap().
>>
>> Reported-by: Stefan Hajnoczi <address@hidden>
>> Signed-off-by: Haozhong Zhang <address@hidden>
> 
> Reviewed-by: Juan Quintela <address@hidden>
> 
> As this function is only used on migration, should I integrate it on my
> next push, or do you want to pull it, Paolo?
> 
> Later, Juan.
> 
> 
>> ---
>>  include/exec/ram_addr.h | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
>> index 73d1bea8b6..cbc797ed05 100644
>> --- a/include/exec/ram_addr.h
>> +++ b/include/exec/ram_addr.h
>> @@ -377,6 +377,7 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
>> *rb,
>>                                                 uint64_t *real_dirty_pages)
>>  {
>>      ram_addr_t addr;
>> +    ram_addr_t offset = rb->offset;
>>      unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
>>      uint64_t num_dirty = 0;
>>      unsigned long *dest = rb->bmap;
>> @@ -386,8 +387,9 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
>> *rb,
>>          int k;
>>          int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
>>          unsigned long * const *src;
>> -        unsigned long idx = (page * BITS_PER_LONG) / 
>> DIRTY_MEMORY_BLOCK_SIZE;
>> -        unsigned long offset = BIT_WORD((page * BITS_PER_LONG) %
>> +        unsigned long word = BIT_WORD((start + offset) >> TARGET_PAGE_BITS);
>> +        unsigned long idx = (word * BITS_PER_LONG) / 
>> DIRTY_MEMORY_BLOCK_SIZE;
>> +        unsigned long offset = BIT_WORD((word * BITS_PER_LONG) %
>>                                          DIRTY_MEMORY_BLOCK_SIZE);

The shadowing between the two variables named offset is a bit ugly.

Please use rb->offset in the initialization of "word", and declare...

>>  
>>          rcu_read_lock();
>> @@ -416,7 +418,7 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock 
>> *rb,
>>      } else {

... "ram_addr_t offset = offset" here.

Paolo

>>          for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
>>              if (cpu_physical_memory_test_and_clear_dirty(
>> -                        start + addr,
>> +                        start + addr + offset,
>>                          TARGET_PAGE_SIZE,
>>                          DIRTY_MEMORY_MIGRATION)) {
>>                  *real_dirty_pages += 1;