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Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty pa

From: Alex Duyck
Subject: Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking
Date: Mon, 6 Jun 2016 09:04:27 -0700

On Mon, Jun 6, 2016 at 2:18 AM, Zhou Jie <address@hidden> wrote:
> Hi Alex,
> On 2016/1/6 0:18, Alexander Duyck wrote:
>> On Tue, Jan 5, 2016 at 1:40 AM, Michael S. Tsirkin <address@hidden> wrote:
>>> On Mon, Jan 04, 2016 at 07:11:25PM -0800, Alexander Duyck wrote:
>>>>>> The two mechanisms referenced above would likely require coordination
>>>>>> with
>>>>>> QEMU and as such are open to discussion.  I haven't attempted to
>>>>>> address
>>>>>> them as I am not sure there is a consensus as of yet.  My personal
>>>>>> preference would be to add a vendor-specific configuration block to
>>>>>> the
>>>>>> emulated pci-bridge interfaces created by QEMU that would allow us to
>>>>>> essentially extend shpc to support guest live migration with
>>>>>> pass-through
>>>>>> devices.
>>>>> shpc?
>>>> That is kind of what I was thinking.  We basically need some mechanism
>>>> to allow for the host to ask the device to quiesce.  It has been
>>>> proposed to possibly even look at something like an ACPI interface
>>>> since I know ACPI is used by QEMU to manage hot-plug in the standard
>>>> case.
>>>> - Alex
>>> Start by using hot-unplug for this!
>>> Really use your patch guest side, and write host side
>>> to allow starting migration with the device, but
>>> defer completing it.
>> Yeah, I'm fully on board with this idea, though I'm not really working
>> on this right now since last I knew the folks on this thread from
>> Intel were working on it.  My patches were mostly meant to be a nudge
>> in this direction so that we could get away from the driver specific
>> code.
> I have seen your email about live migration.
> I conclude the idea you proposed as following.
> 1. Extend swiotlb to allow for a page dirtying functionality.
> 2. Use pci express capability to implement of a PCI bridge to act
>    as a bridge between direct assigned devices and the host bridge.
> 3. Using APCI event or extend shpc driver to support device pause.
> Is it right?
> Will you implement the patchs for live migration?

That is pretty much the heart of the proposal I had.  I submitted an
RFC as a proof-of-concept for item 1 in the hopes that someone else
might try tackling items 2 and 3 but I haven't seen any updates since
then.  The trick is to find a way to make it so that item 1 doesn't
slow down standard SWIOTLB when you are not migrating a VM. If nothing
else we would probably just need to add a static key that we could
default to false unless there is a PCI bridge indicating we are
starting a migration.

I haven't had time to really work on this though. In addition I am not
that familiar with QEMU and the internals of live migration so pieces
2 and 3 would take me some additional time to work on.

- Alex

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