Re: [Qemu-devel] [PATCH RFC] memory: drop _overlap variant

[Michael S. Tsirkin]

On Thu, Feb 14, 2013 at 07:02:15PM +0200, Avi Kivity wrote:
> On Thu, Feb 14, 2013 at 6:50 PM, Michael S. Tsirkin <address@hidden> wrote:
> >> > As you see, ioapic at 0xfec00000 overlaps pci-hole.
> >> > ioapic is guest programmable in theory - should use _overlap?
> >> > pci-hole is not but can overlap with ioapic.
> >> > So also _overlap?
> >>
> >> It's a bug.  The ioapic is in the pci address space, not the system
> >> address space.  And yes it's overlappable.
> >
> > So you want to put it where? Under pci-hole?
> 
> No, under the pci address space.  Look at the 440fx block diagram.
> 
> > And we'll have to teach all machine types
> > creating pci-hole about it?
> 
> No.
> 
> >
> >> >
> >> > Let's imagine someone writes a guest programmable device for
> >> > ARM. Now we should update all ARM devices from regular to _overlap?
> >>
> >> It's sufficient to update the programmable device.
> >
> > Then the device can be higher priority (works for apic)
> > but not lower priority. Make priority signed?
> 
> Is there an actual real problem that needs fixing?

Yes. Guests sometimes cause device BARs to temporary overlap
the APIC range during BAR sizing. It works fine on a physical
system but fails on KVM since pci has same priority.

See the report:
[BUG] Guest OS hangs on boot when 64bit BAR present 

-- 
MST