Re: [Qemu-devel] [PATCH] cpu_physical_memory_sync_dirty_bitmap: Another alignment fix

[Wei Huang <wei@redhat.com>]

On 01/03/2018 12:33 PM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> This code has an optimised, word aligned version, and a boring
> unaligned version. My commit f70d345 fixed one alignment issue, but
> there's another.
> 
> The optimised version operates on 'longs' dealing with (typically) 64
> pages at a time, replacing the whole long by a 0 and counting the bits.
> If the Ramblock is less than 64bits in length that long can contain bits
> representing two different RAMBlocks, but the code will update the
> bmap belinging to the 1st RAMBlock only while having updated the total
> dirty page count for both.
> 
> This probably didn't matter prior to 6b6712ef which split the dirty
> bitmap by RAMBlock, but now they're separate RAMBlocks we end up
> with a count that doesn't match the state in the bitmaps.
> 
> Symptom:
>   Migration showing a few dirty pages left to be sent constantly
>   Seen on aarch64 and x86 with x86+ovmf
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> Reported-by: Wei Huang <wei@redhat.com>
> Fixes: 6b6712efccd383b48a909bee0b29e079a57601ec

This solves the failure I saw in the migration test case.

Acked-by: Wei Huang <wei@redhat.com>

> ---
>  include/exec/ram_addr.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index 6cbc02aa0f..7633ef6342 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -391,9 +391,10 @@ uint64_t cpu_physical_memory_sync_dirty_bitmap(RAMBlock *rb,
>      uint64_t num_dirty = 0;
>      unsigned long *dest = rb->bmap;
>  
> -    /* start address is aligned at the start of a word? */
> +    /* start address and length is aligned at the start of a word? */
>      if (((word * BITS_PER_LONG) << TARGET_PAGE_BITS) ==
> -         (start + rb->offset)) {
> +         (start + rb->offset) &&
> +        !(length & ((BITS_PER_LONG << TARGET_PAGE_BITS) - 1))) {
>          int k;
>          int nr = BITS_TO_LONGS(length >> TARGET_PAGE_BITS);
>          unsigned long * const *src;
>