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

[Juan Quintela <quintela@redhat.com>]

"Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> 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

Reviewed-by: Juan Quintela <quintela@redhat.com>