Re: [Bug] Intel RMRR support with upstream Qemu

[Alexey G <x1917x@gmail.com>]

> On Fri, 21 Jul 2017 10:57:55 +0000
> "Zhang, Xiong Y" <xiong.y.zhang@intel.com> wrote:
> 
> > On an intel skylake machine with upstream qemu, if I add
> > "rdm=strategy=host, policy=strict" to hvm.cfg, win 8.1 DomU couldn't
> > boot up and continues reboot.
> > 
> > Steps to reproduce this issue:
> > 
> > 1)       Boot xen with iommu=1 to enable iommu
> > 2)       hvm.cfg contain:
> > 
> > builder="hvm"
> > 
> > memory=xxxx
> > 
> > disk=['win8.1 img']
> > 
> > device_model_override='qemu-system-i386'
> > 
> > device_model_version='qemu-xen'
> > 
> > rdm="strategy=host,policy=strict"
> > 
> > 3)       xl cr hvm.cfg
> > 
> > Conditions to reproduce this issue:
> > 
> > 1)       DomU memory size > the top address of RMRR. Otherwise, this
> > issue will disappear.
> > 2)       rdm=" strategy=host,policy=strict" should exist
> > 3)       Windows DomU.  Linux DomU doesn't have such issue.
> > 4)       Upstream qemu.  Traditional qemu doesn't have such issue.
> > 
> > In this situation, hvmloader will relocate some guest ram below RMRR to
> > high memory, and it seems window guest access an invalid address. Could
> > someone give me some suggestions on how to debug this ?  
> 
> You're likely have RMRR range(s) below 2GB boundary.
> 
> You may try the following:
> 
> 1. Specify some large 'mmio_hole' value in your domain configuration file,
> ex. mmio_hole=2560
> 2. If it won't help, 'xl dmesg' output might come useful
> 
> Right now upstream QEMU still doesn't support relocation of parts
> of guest RAM to >4GB boundary if they were overlapped by MMIO ranges.
> AFAIR forcing allow_memory_relocate to 1 for hvmloader didn't bring
> anything good for HVM guest.
> 
> Setting the mmio_hole size manually allows to create a "predefined"
> memory/MMIO hole layout for both QEMU (via 'max-ram-below-4g') and
> hvmloader (via a XenStore param), effectively avoiding MMIO/RMRR overlaps
> or RAM relocation in hvmloader, so this might help.

Wrote too soon, "policy=strict" means that you won't be able to create a
DomU if RMRR was below 2G... so it's actually should be above 2GB. Anyway,
try setting mmio_hole size.

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