Re: [Qemu-devel] [PATCH 00/17] RFC: userfault v2

[zhanghailiang <zhang.zhanghailiang@huawei.com>]

On 2014/10/31 11:29, zhanghailiang wrote:
> On 2014/10/31 10:23, Peter Feiner wrote:
>> On Thu, Oct 30, 2014 at 07:31:48PM +0800, zhanghailiang wrote:
>>> On 2014/10/30 1:46, Andrea Arcangeli wrote:
>>>> On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote:
>>>>> I want to confirm a question:
>>>>> Can we support distinguishing between writing and reading memory for userfault?
>>>>> That is, we can decide whether writing a page, reading a page or both trigger userfault.
>>>> Mail is going to be long enough already so I'll just assume tracking
>>>> dirty memory in userland (instead of doing it in kernel) is worthy
>>>> feature to have here.
>>
>> I'll open that can of worms :-)
>>
>>> [...]
>>> Er, maybe i didn't describe clearly. What i really need for live memory snapshot
>>> is only wrprotect fault, like kvm's dirty tracing mechanism, *only tracing write action*.
>>>
>>> So, what i need for userfault is supporting only wrprotect fault. i don't
>>> want to get notification for non present reading faults, it will influence
>>> VM's performance and the efficiency of doing snapshot.
>>
>> Given that you do care about performance Zhanghailiang, I don't think that a
>> userfault handler is a good place to track dirty memory. Every dirtying write
>> will block on the userfault handler, which is an expensively slow proposition
>> compared to an in-kernel approach.
>>
>
> Agreed, but for doing live memory snapshot (VM is running when do snapsphot),
> we have to do this (block the write action), because we have to save the page before it
> is dirtied by writing action. This is the difference, compared to pre-copy migration.
>

Again;) For snapshot, i don't use its dirty tracing ability, i just use it to block write action,
and save page, and then i will remove its write protect.

>>> Also, i think this feature will benefit for migration of ivshmem and vhost-scsi
>>> which have no dirty-page-tracing now.
>>
>> I do agree wholeheartedly with you here. Manually tracking non-guest writes
>> adds to the complexity of device emulation code. A central fault-driven means
>> for dirty tracking writes from the guest and host would be a welcome
>> simplification to implementing pre-copy migration. Indeed, that's exactly what
>> I'm working on! I'm using the softdirty bit, which was introduced recently for
>> CRIU migration, to replace the use of KVM's dirty logging and manual dirty
>> tracking by the VMM during pre-copy migration. See
>
> Great! Do you plan to issue your patches to community? I mean is your work based on
> qemu? or an independent tool (CRIU migration?) for live-migration?
> Maybe i could fix the migration problem for ivshmem in qemu now,
> based on softdirty mechanism.
>
>> Documentation/vm/soft-dirty.txt and pagemap.txt in case you aren't familiar. To
>
> I have read them cursorily, it is useful for pre-copy indeed. But it seems that
> it can not meet my need for snapshot.
>
>> make softdirty usable for live migration, I've added an API to atomically
>> test-and-clear the bit and write protect the page.
>
> How can i find the API? Is it been merged in kernel's master branch already?
>
>
> Thanks,
> zhanghailiang
>
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