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

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Date:	Tue, 5 Jan 2016 11:03:38 +0000
From:	"Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Alexander Duyck <alexander.duyck@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Alexander Duyck <aduyck@...antis.com>, kvm@...r.kernel.org,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	x86@...nel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	qemu-devel@...gnu.org, Lan Tianyu <tianyu.lan@...el.com>,
	Yang Zhang <yang.zhang.wz@...il.com>,
	Alexander Graf <agraf@...e.de>,
	Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [RFC PATCH 0/3] x86: Add support for guest DMA dirty page
 tracking

* Michael S. Tsirkin (mst@...hat.com) wrote:
> On Tue, Jan 05, 2016 at 10:45:25AM +0000, Dr. David Alan Gilbert wrote:
> > * Michael S. Tsirkin (mst@...hat.com) wrote:
> > > On Tue, Jan 05, 2016 at 10:01:04AM +0000, Dr. David Alan Gilbert wrote:
> > > > * Michael S. Tsirkin (mst@...hat.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.
> > > > > 
> > > > > 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
> > > > > 
> > > > > 
> > > > > 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.
> > > > > 
> > > > > 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".
> > > > 
> > > > 
> > > > Doing a hot-unplug is going to upset the guests network stacks view
> > > > of the world; that's something we don't want to change.
> > > > 
> > > > Dave
> > > 
> > > It might but if you store the IP and restore it quickly
> > > after migration e.g. using guest agent, as opposed to DHCP,
> > > then it won't.
> > 
> > I thought if you hot-unplug then it will lose any outstanding connections
> > on that device.
> > 
> > > It allows calming the device down in a generic way,
> > > specific drivers can then implement the fast quiesce.
> > 
> > Except that if it breaks the guest networking it's useless.
> > 
> > Dave
> 
> Is hot unplug useless then?

As a migration hack, yes, unless it's paired with a second network device
as a redundent route.
To do what's being suggested here it's got to be done at the device level
and not visible to the networking stack.

Dave

> 
> > > 
> > > > > 
> > > > > -- 
> > > > > MST
> > > > --
> > > > Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
> > --
> > Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
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