Re: [Qemu-devel] io_mem_notdirty and live migration

[Hollis Blanchard <hollis_blanchard@mentor.com>]

On 03/23/2016 09:53 AM, Paolo Bonzini wrote:
> On 23/03/2016 17:47, Hollis Blanchard wrote:
>> Paolo, is it true that only TB-invalidating writes go through the
>> io_mem_notdirty path? I'm looking at the live migration code now, and it
>> seems like every memory write will go through that path when global
>> dirty memory logging is enabled.
> When live migration is enabled, writes to clean memory (almost all of
> them) will go through that path indeed.  Some writes to the framebuffer
> will go through that path too.
>
> It depends on
>
>        cpu_physical_memory_is_clean(
>                          memory_region_get_ram_addr(section->mr) + xlat))
>
> in tlb_set_page_with_attrs.

I'm guessing that when live migration starts (ram_save_setup), the TLB 
must be flushed so that new entries can be created with the TLB_NOTDIRTY 
flag. Otherwise, pre-migration entries without TLB_NOTDIRTY flag could 
live on, allowing the TBs to directly modify guest RAM without tracking, 
right?

I can't find anything underneath ram_save_setup() that does this, 
though. Am I just missing it?

-- 
Hollis Blanchard <hollis_blanchard@mentor.com>
Mentor Graphics Emulation Division