Re: [Qemu-devel] [PATCH 2/3] migration/ram.c: start of cpu_physical_memory_sync_dirty_bitmap is always 0

[Paolo Bonzini]

On 17/07/19 03:13, Wei Yang wrote:
> On Tue, May 14, 2019 at 03:21:08PM +0100, Dr. David Alan Gilbert wrote:
>> * Wei Yang (address@hidden) wrote:
>>> Since start of cpu_physical_memory_sync_dirty_bitmap is always 0, we can
>>> remove this parameter and simplify the calculation a bit.
>>>
>>> Signed-off-by: Wei Yang <address@hidden>
>>
>> So I think you're right it's currently unused; however, lets ask
>> Paolo: Do we need to keep this parameter for flexiblity?
>>
> 
> Hey, Paolo
> 
> Do you have some spare time to give some insight?

I think it's cleaner to leave the start argument in place.  However,
I'll note that in migration_bitmap_sync_range the length argument is
always block->used_length so that's also unnecessary (you already have
block).

Paolo

>> Dave
>>
>>> ---
>>>  include/exec/ram_addr.h | 15 ++++++---------
>>>  migration/ram.c         |  2 +-
>>>  2 files changed, 7 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
>>> index 9ecd911c3e..3dfb2d52fb 100644
>>> --- a/include/exec/ram_addr.h
>>> +++ b/include/exec/ram_addr.h
>>> @@ -409,18 +409,16 @@ static inline void 
>>> cpu_physical_memory_clear_dirty_range(ram_addr_t start,
>>>  
>>>  static inline
>>>  uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>>> -                                               ram_addr_t start,
>>>                                                 ram_addr_t length,
>>>                                                 uint64_t *real_dirty_pages)
>>>  {
>>>      ram_addr_t addr;
>>> -    unsigned long word = BIT_WORD((start + rb->offset) >> 
>>> TARGET_PAGE_BITS);
>>> +    unsigned long word = BIT_WORD(rb->offset >> TARGET_PAGE_BITS);
>>>      uint64_t num_dirty = 0;
>>>      unsigned long *dest = rb->bmap;
>>>  
>>> -    /* start address and length is aligned at the start of a word? */
>>> -    if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) ==
>>> -         (start + rb->offset) &&
>>> +    /* offset and length is aligned at the start of a word? */
>>> +    if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) == (rb->offset) &&
>>>          !(length & ((BITS_PER_LONG << TARGET_PAGE_BITS) - 1))) {
>>>          int k;
>>>          int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
>>> @@ -428,14 +426,13 @@ uint64_t 
>>> cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>>>          unsigned long idx = (word * BITS_PER_LONG) / 
>>> DIRTY_MEMORY_BLOCK_SIZE;
>>>          unsigned long offset = BIT_WORD((word * BITS_PER_LONG) %
>>>                                          DIRTY_MEMORY_BLOCK_SIZE);
>>> -        unsigned long page = BIT_WORD(start >> TARGET_PAGE_BITS);
>>>  
>>>          rcu_read_lock();
>>>  
>>>          src = atomic_rcu_read(
>>>                  &ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION])->blocks;
>>>  
>>> -        for (k = page; k < page + nr; k++) {
>>> +        for (k = 0; k < nr; k++) {
>>>              if (src[idx][offset]) {
>>>                  unsigned long bits = atomic_xchg(&src[idx][offset], 0);
>>>                  unsigned long new_dirty;
>>> @@ -458,11 +455,11 @@ uint64_t 
>>> cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>>>  
>>>          for (addr = 0; addr < length; addr += TARGET_PAGE_SIZE) {
>>>              if (cpu_physical_memory_test_and_clear_dirty(
>>> -                        start + addr + offset,
>>> +                        addr + offset,
>>>                          TARGET_PAGE_SIZE,
>>>                          DIRTY_MEMORY_MIGRATION)) {
>>>                  *real_dirty_pages += 1;
>>> -                long k = (start + addr) >> TARGET_PAGE_BITS;
>>> +                long k = addr >> TARGET_PAGE_BITS;
>>>                  if (!test_and_set_bit(k, dest)) {
>>>                      num_dirty++;
>>>                  }
>>> diff --git a/migration/ram.c b/migration/ram.c
>>> index 9948b2d021..1def8122e9 100644
>>> --- a/migration/ram.c
>>> +++ b/migration/ram.c
>>> @@ -1646,7 +1646,7 @@ static void migration_bitmap_sync_range(RAMState *rs, 
>>> RAMBlock *rb,
>>>                                          ram_addr_t length)
>>>  {
>>>      rs->migration_dirty_pages +=
>>> -        cpu_physical_memory_sync_dirty_bitmap(rb, 0, length,
>>> +        cpu_physical_memory_sync_dirty_bitmap(rb, length,
>>>                                                &rs->num_dirty_pages_period);
>>>  }
>>>  
>>> -- 
>>> 2.19.1
>>>
>> --
>> Dr. David Alan Gilbert / address@hidden / Manchester, UK
>