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

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  Re: Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA  dirty page tracking Zhou Jie <zhoujie2011@cn.fujitsu.com> - 2016-06-06 11:20 +0200
    Re: Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA  dirty page tracking Alex Duyck <aduyck@mirantis.com> - 2016-06-06 18:10 +0200
      Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty  page tracking Zhou Jie <zhoujie2011@cn.fujitsu.com> - 2016-06-09 12:20 +0200
        Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty  page tracking Alexander Duyck <alexander.duyck@gmail.com> - 2016-06-09 17:40 +0200

#1414813 — Re: Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking

Hi Alex,

On 2016/1/6 0:18, Alexander Duyck wrote:
> On Tue, Jan 5, 2016 at 1:40 AM, Michael S. Tsirkin <mst@redhat.com> 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?

Sincerely,
Zhou Jie


>
>> So
>>
>> 1.- host tells guest to start tracking memory writes
>> 2.- guest acks
>> 3.- migration starts
>> 4.- most memory is migrated
>> 5.- host tells guest to eject device
>> 6.- guest acks
>> 7.- stop vm and migrate rest of state
>>
>
> Sounds about right.  The only way this differs from what I see as the
> final solution for this is that instead of fully ejecting the device
> in step 5 the driver would instead pause the device and give the host
> something like 10 seconds to stop the VM and resume with the same
> device connected if it is available.  We would probably also need to
> look at a solution that would force the device to be ejected or abort
> prior to starting the migration if it doesn't give us the ack in step
> 2.
>
>> It will already be a win since hot unplug after migration starts and
>> most memory has been migrated is better than hot unplug before migration
>> starts.
>
> Right.  Generally the longer the VF can be maintained as a part of the
> guest the longer the network performance is improved versus using a
> purely virtual interface.
>
>> Then measure downtime and profile. Then we can look at ways
>> to quiesce device faster which really means step 5 is replaced
>> with "host tells guest to quiesce device and dirty (or just unmap!)
>> all memory mapped for write by device".
>
> Step 5 will be the spot where we really need to start modifying
> drivers.  Specifically we probably need to go through and clean-up
> things so that we can reduce as many of the delays in the driver
> suspend/resume path as possible.  I suspect there is quite a bit that
> can be done there that would probably also improve boot and shutdown
> times since those are also impacted by the devices.
>
> - Alex
>
>
>
> .
>

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#1415253

On Mon, Jun 6, 2016 at 2:18 AM, Zhou Jie <zhoujie2011@cn.fujitsu.com> wrote:
> Hi Alex,
>
>
> On 2016/1/6 0:18, Alexander Duyck wrote:
>>
>> On Tue, Jan 5, 2016 at 1:40 AM, Michael S. Tsirkin <mst@redhat.com> 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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#1418222 — Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking

TO Alex
TO Michael

    In your solution you add a emulate PCI bridge to act as
    a bridge between direct assigned devices and the host bridge.
    Do you mean put all direct assigned devices to
    one emulate PCI bridge?
    If yes, this maybe bring some problems.

    We are writing a patchset to support aer feature in qemu.
    When assigning a vfio device with AER enabled, we must check whether
    the device supports a host bus reset (ie. hot reset) as this may be
    used by the guest OS in order to recover the device from an AER
    error.
    QEMU must therefore have the ability to perform a physical
    host bus reset using the existing vfio APIs in response to a virtual
    bus reset in the VM.
    A physical bus reset affects all of the devices on the host bus.
    Therefore all physical devices affected by a bus reset must be
    configured on the same virtual bus in the VM.
    And no devices unaffected by the bus reset,
    be configured on the same virtual bus.

    http://lists.nongnu.org/archive/html/qemu-devel/2016-05/msg02989.html

Sincerely,
Zhou Jie

On 2016/6/7 0:04, Alex Duyck wrote:
> On Mon, Jun 6, 2016 at 2:18 AM, Zhou Jie <zhoujie2011@cn.fujitsu.com> wrote:
>> Hi Alex,
>>
>>
>> On 2016/1/6 0:18, Alexander Duyck wrote:
>>>
>>> On Tue, Jan 5, 2016 at 1:40 AM, Michael S. Tsirkin <mst@redhat.com> 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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#1418437 — Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking

On Thu, Jun 9, 2016 at 3:14 AM, Zhou Jie <zhoujie2011@cn.fujitsu.com> wrote:
> TO Alex
> TO Michael
>
>    In your solution you add a emulate PCI bridge to act as
>    a bridge between direct assigned devices and the host bridge.
>    Do you mean put all direct assigned devices to
>    one emulate PCI bridge?
>    If yes, this maybe bring some problems.
>
>    We are writing a patchset to support aer feature in qemu.
>    When assigning a vfio device with AER enabled, we must check whether
>    the device supports a host bus reset (ie. hot reset) as this may be
>    used by the guest OS in order to recover the device from an AER
>    error.
>    QEMU must therefore have the ability to perform a physical
>    host bus reset using the existing vfio APIs in response to a virtual
>    bus reset in the VM.
>    A physical bus reset affects all of the devices on the host bus.
>    Therefore all physical devices affected by a bus reset must be
>    configured on the same virtual bus in the VM.
>    And no devices unaffected by the bus reset,
>    be configured on the same virtual bus.
>
>    http://lists.nongnu.org/archive/html/qemu-devel/2016-05/msg02989.html
>
> Sincerely,
> Zhou Jie

That makes sense, but I don't think you have to worry much about this
at this point at least on my side as this was mostly just theory and I
haven't had a chance to put any of it into practice as of yet.

My idea has been evolving on this for a while.  One thought I had is
that we may want to have something like an emulated IOMMU and if
possible we would want to split it up over multiple domains just so we
can be certain that the virtual interfaces and the physical ones
existed in separate domains.  In regards to your concerns perhaps what
we could do is put each assigned device into its own domain to prevent
them from affecting each other.  To that end we could probably break
things up so that each device effectively lives in its own PCIe slot
in the emulated system.  Then when we start a migration of the guest
the assigned device domains would then have to be tracked for unmap
and sync calls when the direction is from the device.

I will keep your concerns in mind in the future when I get some time
to look at exploring this solution further.

- Alex

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