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

From: Zhou Jie
Subject: Re: [Qemu-devel] [RFC PATCH 0/3] x86: Add support for guest DMA dirty page tracking
Date: Thu, 9 Jun 2016 18:14:40 +0800
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.1.0

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
   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.


Zhou Jie

On 2016/6/7 0:04, Alex Duyck wrote:
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
QEMU and as such are open to discussion.  I haven't attempted to
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
emulated pci-bridge interfaces created by QEMU that would allow us to
essentially extend shpc to support guest live migration with


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

- 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

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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