Re: [PATCH] kvm: Take into account the unaligned section size when preparing bitmap

[Peter Xu <peterx@redhat.com>]

On Mon, Dec 14, 2020 at 10:14:11AM +0800, zhukeqian wrote:

[...]

> >>> Though indeed I must confess I don't know how it worked in general when host
> >>> page size != target page size, at least for migration.  For example, I believe
> >>> kvm dirty logging is host page size based, though migration should be migrating
> >>> pages in guest page size granule when it spots a dirty bit set.

[1]

> Hi Peter,

Keqian,

> > OTOH I'm more worried on the other question on how we handle guest psize !=
> > host psize case for migration now...
> I think it does not matter when guest_psize != host_psize, as we only need to interact with
> stage2 page tables during migration. Stage2 is enough to tracking guest dirty memory, and even
> if guest close stage1, we also can do a successful migration.

I don't know why 2-stage matters here, since I believe KVM can track dirty
pages either using two dimentional paging or shadowing, however it's always
done in host small page size.  The question I'm confused is there seems to have
a size mismatch between qemu migration and what kvm does [1].  For example, how
migration works on ARM64 where host has psize==4K while guest has psize=64K.

Thanks,

-- 
Peter Xu