Re: [Qemu-devel] [PATCH 1/1] Count used RAMBlock pages for migration_dirty_pages

[Paolo Bonzini <pbonzini@redhat.com>]

Il 21/03/2014 14:22, Dr. David Alan Gilbert ha scritto:
> I don't think they overlap, but I worry that the end of one block
> and the start of the next might be on the same page.
> The code that got me worried was migration_bitmap_sync_range
> that seemd to be general; but actually that's worrying about 64bit words
> not pages.
> What happens with things like '/rom@etc/table-loader' which is only
> 4k on x86 when they are on boxes with bigger target_page.

Do you mean bigger host page?

RAM sizes are always rounded up to target page size:

     size = TARGET_PAGE_ALIGN(size);

If I understand correctly all that matters here is ram addresses have no 
remainder WRT target page sizes.

Host page sizes matter only for KVM (see hpratio in 
cpu_physical_memory_set_dirty_lebitmap in include/exec/ram_addr.h).  In 
this case dirtying a 4K area also dirties 12K of adjacent ram_addr_t. 
Things could go wrong if those ram_addr_t's are not part of any 
RAMBlock, because in that case you will loop endlessly (I think).  But 
this is not something your patch introduces.

So it looks like your patch could also fix the problem Juan reported at 
http://article.gmane.org/gmane.comp.emulators.qemu/247462 -- but perhaps 
only on hosts where !KVM || TARGET_PAGE_SIZE==getpagesize().

Paolo