Re: [Qemu-devel] Re: [PATCH 2/3] qemu: make cirrus init value pci spec compliant

[Gleb Natapov]

On Fri, Oct 09, 2009 at 08:43:59AM +0200, Michael S. Tsirkin wrote:
> On Thu, Oct 08, 2009 at 07:40:12PM +0100, Jamie Lokier wrote:
> > Avi Kivity wrote:
> > > On 10/08/2009 06:06 PM, Michael S. Tsirkin wrote:
> > > >On Thu, Oct 08, 2009 at 05:29:29PM +0200, Avi Kivity wrote:
> > > >   
> > > >>On 10/08/2009 04:52 PM, Michael S. Tsirkin wrote:
> > > >>     
> > > >>>PCI memory should be disabled at reset, otherwise
> > > >>>we might claim transactions at address 0.
> > > >>>I/O should also be disabled, although for cirrus
> > > >>>it is harmless to enable it as we do not
> > > >>>have I/O bar.
> > > >>>
> > > >>>Note: need bios fix for this patch to work:
> > > >>>currently pc-bios incorrently assumes that it does not
> > > >>>need to enable i/o unless device has i/o bar.
> > > >>>
> > > >>>Signed-off-by: Michael S. Tsirkin<address@hidden>
> > > >>
> > > >>This needs to be conditional on the machine type.  Old machines must
> > > >>retain this code for live migration to work (we need to live migrate the
> > > >>bios, so we can't assume the bios fix is in during live migration from
> > > >>older qemus).
> > > >>     
> > > >No, if you migrate from older qemu you will be fine as command
> > > >is enabled there on init.
> > > >   
> > > 
> > > Right.
> > 
> > No, I think Avi was right the first time.
> > 
> > Migrating from an older qemu will be fine at first, but at the next
> > reset _following_ migration, it'll be running the old BIOS on a new
> > qemu and fail.
> > 
> > -- Jamie
> 
> I think this just means that there's another bug.
> On reset BIOS should be re-read from flash.
> qemu instead keeps it in RAM, this means that if
> guest corrupts it, or BIOS itself runs self-modifying
> code (which is not uncommon btw), bad things happen.
> 
How do you know it is not uncommon (or happens at all?)
If BIOS is not shadowed it runs directly from ROM. Hard to use
self-modifying code there.

> We should fix qemu to re-read bios from flash.
> Makes sense?
> 
We have option_rom_setup_reset() already. Don't we use it for BIOSes
too?

> More long-term, we have duplication between reset and init
> routines. Maybe devices really should have init and cleanup,
> and on reset we'd cleanup all devices and then init them again?
> There are cases where state needs to be persistent across
> resets (VPD comes to mind) but these are rare,
> and probably need to be backed by writing to file anyway?
> 
> 
> -- 
> MST
> 
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--
                        Gleb.