Re: [Qemu-devel] host physical address width issues/questions for x86_64

[Alex Williamson <alex.williamson@redhat.com>]

On Fri, 13 Oct 2017 18:01:44 +0100
"Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:

> * Prasad Singamsetty (prasad.singamsetty@oracle.com) wrote:
> > Hi,
> > 
> > I am new to the alias. I have some questions on this subject
> > and seek some clarifications from the experts in the team.
> > I ran into a couple of issues when I tried with large configuration
> > ( >= 1TB memory, > 255 CPUs) for x86_64 guest machine.
> > 
> > 1. QEMU uses the default value of 40 (TCG_PHYS_ADDR_BITS) for address
> >    width if user has not specified phys-bits or host-phys-bits=true
> >    property. The default value is obviously not sufficient and
> >    causing guest kernel to crash if configured with >= 1TB
> >    memory. Depending on the linux kernel version in the guest the
> >    panic was in different code paths. The workaround is for the
> >    user to specify the phys-bits property or set the property
> >    host-phys-bits=true.
> > 
> >    QUESTIONS:
...
> > 2. host_address_width in DMAR table structure
> > 
> >    In this case, the default value is set to 39
> >    (VTD_HOST_ADDRESS_WIDTH - 1). With interrupt remapping
> >    enabled for the intel iommu and the guest is configured
> >    with > 255 cpus and >= 1TB memory, the guest kernel hangs
> >    during boot up. This need to be fixed.
> > 
> >    QUESTION:
> >    The question here again is can we fix this to use the
> >    real address width from the host as the default?  
> 
> I don't know DMAR stuff; chatting to Alex (cc'd) it does sound
> like that's an ommission that should be fixed.

[CC +Peter]

On physical hardware VT-d supports either 39 or 48 bit address widths
and generally you'd expect a sufficiently capable IOMMU to be matched
with the CPU.  Seems QEMU has only implemented a lower bit width and
it should probably be forcing phys bits of the VM to 39 to match until
the extended width can be implemented.  Thanks,

Alex