Re: [Qemu-devel] [RFC/PATCH] migration: SMRAM dirty bitmap not fetched from kvm-kmod and not send to destination

["Herongguang (Stephen)" <herongguang.he@huawei.com>]

On 2016/9/22 21:16, Herongguang (Stephen) wrote:
>
>
> On 2016/9/14 17:05, Paolo Bonzini wrote:
>>
>>
>> On 14/09/2016 09:55, Herongguang (Stephen) wrote:
>>> Hi,
>>> We found a problem that when a redhat 6 VM reboots (in grub countdown
>>> UI), migrating this VM will result in VM’s memory difference between
>>> source and destination side. The difference always resides in GPA
>>> 0xA0000~0xC0000, i.e. SMRAM area.
>>>
>>> Occasionally this result in VM instruction emulation error in
>>> destination side.
>>>
>>> After some digging, I think this is because in migration code, in
>>> migration_bitmap_sync(), only memory slots in address space
>>> address_space_memory’s dirty bitmap  fetched from kvm-kmod, while SMRAM
>>> memory slot, in address space smram_address_space’s dirty bitmap not
>>> fetched from kvm-kmod, thus modifications in SMRAM in source side are
>>> not sent to destination side.
>>>
>>> I tried following patch, and this phenomenon does not happen anymore. Do
>>> you think this patch is OK or do you have better idea? Thanks.
>>
>> Nice caatch!
>>
>> I think the right solution here is to sync all RAM memory regions
>> instead of the address spaces.  You can do that by putting a notifier in
>> MemoryRegion; register the notifier in all the RAM creation functions
>> (basically after every mr->ram=true or mr->rom_device=true), and
>> unregister it in memory_region_destructor_ram.
>>
>> Thanks,
>>
>> Paolo
>>
>
> I have some concern:
> 1. For example, vhost does not know about as_id, I wonder if guests in SMM can operate disk or ether card, as in
> that case vhost would not logging dirty pages correctly, without knowing as_id.
>
> 2. If a memory region is disabled/enabled/disabled frequently, since disabled memory regions would be removed
> from memory slots in kvm-kmod, dirty pages would be discarded in kvm-kmod and qemu when disabled, thus missing.
> Is my assumption correct?
After reviewing code, I think question 2 does not exist as qemu will sync dirty page before removing memory slots in kvm_set_phys_mem.

>
> 3. I agree your opinion that the right solution is to get dirty-page information for all memory region from
> kvm-kmod. But I found it’s somewhat hard to implement since kvm_log_sync() expects a MemoryRegionSection*
> parameter. Do you have good idea?
>
> As to all the ram memory regions, I think they are all in the ram_list.blocks, so there is no need to create
> a notifier, is this correct?