Re: [PATCH v2 5/6] migration/postcopy: Handle RAMBlocks with a RamDiscardManager on the destination

[David Hildenbrand]

On 24.07.21 00:10, Peter Xu wrote:
> On Fri, Jul 23, 2021 at 09:01:42PM +0200, David Hildenbrand wrote:
> > It can happen in corner cases and is valid: with the current virtio-mem
> > spec, guests are allowed to read unplugged memory. This will, for example,
> > happen on older Linux guests when reading /proc/kcore or (with even older
> > guests) when dumping guest memory via kdump. These corner cases were the
> > main reason why the spec allows for it -- until we have guests properly
> > adjusted such that it won't happen even in corner cases.
> >
> > A future feature bit will disallow it for the guest: required for supporting
> > shmem/hugetlb cleanly. With that in place, I agree that we would want to
> > warn in this case!
>
> OK that makes sense; with the page_size change, feel free to add:

I just realized that relying on the page_size would be wrong.

We migrate TARGET_PAGE_SIZE chunks and the offset might not be page_size 
aligned. So if we were to replace TARGET_PAGE_SIZE by rb->page_size, we 
might accidentally cover a "too big" range.

Makes sense?

> Reviewed-by: Peter Xu <peterx@redhat.com>


--
Thanks,

David / dhildenb