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

[zhukeqian]

On 2020/12/14 23:36, Peter Xu wrote:
> 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.
> 
Hi Peter,

OK, I got it ;-) Do you mean qemu_real_host_page_size != TARGET_PAGE_SIZE?
After my analysis, I see that when qemu_real_host_page_size != TARGET_PAGE_SIZE,
there are some problems indeed. I have send out some patches, please check 
whether they solve this
problem, thanks!

Keqian.

> Thanks,
>