Re: [Xen-devel] [PATCH v4 0/6] save/restore on Xen

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

On 01/23/2012 11:13 AM, Jan Kiszka wrote:
> > > > To reply to your previous question more clearly: at restore time Qemu on
> > > > Xen would run in a non-standard scenario; the restore of the RAM happens
> > > > before QEMU is even started.
> > > >
> > > > That is unfortunate but it would be very hard to change (I can give you
> > > > more details if you are interested in the reasons why it would be so
> > > > difficult).
> > > If you can't change this, you need to properly introduce this new
> > > scenario - pre-initialized RAM - to the QEMU device model. Or you will
> > > see breakage outside cirrus sooner or later as well. So it might be good
> > > to explain the reason why it can't be changed under Xen when motivating
> > > this concept extension to QEMU.
> > OK.
> > Are you thinking about introducing this concept as a new runstate?
> > This special runstate could be set at restore time only on Xen.
> >
> >
> > BTW the main reasons for having Xen saving the RAM are:
> >
> > - the need to support PV guests, that often run without Qemu;
> > - the current save format, that is built around the fact that Xen saves the 
> > memory;
> > - the fact that Qemu might be running in a very limited stub-domain.
> Your problem is not the fact that guest RAM is restored by an external
> component. Your problem is that QEMU has no control over the when. If
> you fix this, you could coordinate the restoring with the initial device
> reset and would solve all potential current and future issues, not only
> this single cirrus related one.
Generally speaking, RAM is an independent device in most useful cases.  Onboard 
RAM is a very special case because it's extremely unusual.
But since some video cards can make use of dedicated external RAM, I don't think 
any video card really depends on initial RAM state.
What's most likely here is that the VGA BIOS of a Cirrus card sets an initial 
RAM state during device initialization.
We really should view RAM as just another device so I don't like the idea of 
propagating a global concept of "when RAM is restored" because that treats it 
specially compared to other devices.
But viewing RAM as just another device, having Xen only restore a subset of 
devices should be a reasonable thing to do moving forward.  The main problem 
here I believe is that we have part of the VGA Bios functionality in the 
hardware emulation.
Regards,

Anthony Liguori

> Jan

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