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Re: [Qemu-devel] host physical address width issues/questions for x86_64
From: |
Alex Williamson |
Subject: |
Re: [Qemu-devel] host physical address width issues/questions for x86_64 |
Date: |
Fri, 13 Oct 2017 11:14:03 -0600 |
On Fri, 13 Oct 2017 18:01:44 +0100
"Dr. David Alan Gilbert" <address@hidden> wrote:
> * Prasad Singamsetty (address@hidden) 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
- [Qemu-devel] host physical address width issues/questions for x86_64, Prasad Singamsetty, 2017/10/13
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Dr. David Alan Gilbert, 2017/10/13
- Re: [Qemu-devel] host physical address width issues/questions for x86_64,
Alex Williamson <=
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Peter Xu, 2017/10/14
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Prasad Singamsetty, 2017/10/16
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Peter Xu, 2017/10/16
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Fam Zheng, 2017/10/18
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Prasad Singamsetty, 2017/10/18
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Peter Xu, 2017/10/18
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Prasad Singamsetty, 2017/10/20
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Peter Xu, 2017/10/23
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Prasad Singamsetty, 2017/10/23
- Re: [Qemu-devel] host physical address width issues/questions for x86_64, Peter Xu, 2017/10/26