On 10/16/2017 8:56 PM, Peter Xu wrote:
On Mon, Oct 16, 2017 at 10:02:25AM -0700, Prasad Singamsetty wrote:
On 10/14/2017 8:53 PM, Peter Xu wrote:
On Fri, Oct 13, 2017 at 11:14:03AM -0600, Alex Williamson wrote:
On Fri, 13 Oct 2017 18:01:44 +0100
"Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote:
* Prasad Singamsetty (prasad.singamse...@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
There were patches that tried to enable 48 bits GAW but it was
not accepted somehow:
https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg01886.html
Would this help in any way?
Thanks Alex for the patch info. Just curious why the patch was not
accepted. Any way, I will try it.
I don't sure I know the reason. Anyway, it originated from one of
Fam's request for some NVMe tests. If it can really help for your use
case as well, please feel free to revive those patches, or let me know
so that I can respin. Thanks,
Thanks Peter. I will start with your patch and see if I can get
it to work first.
A quick question. Looking at the code, it doesn't look like there
is a way to disable dma remapping. User may have a case where he
is interested only in interrupt remapping (for > 255 cpus) and
not DMA remapping. Is that scenario considered before?
Thanks.
--Prasad
