xen: Disable highmem on qemuarm

By default, highmem option is enabled for machine types later than
virt-2.12. This allows qemu to place devices and RAM in physical
address space above 32-bits. This can cause issues as according to the
documentation Xen supports up to 12GiB of physical address space.
Recently the issue was observed using runqemu, that was causing Linux
running on top of Xen to stall when trying to access ECAM space placed
by qemu at 256GiB mark. Even though this issue is most probably related
to QEMU and not Xen (the investigation showed that it can map ECAM
space correctly), it is best to avoid such issues by disabling highmem
on qemuarm.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/classes/qemuboot-xen-defaults.bbclass b/classes/qemuboot-xen-defaults.bbclass
index f1550ccc..460cbe6a 100644
--- a/classes/qemuboot-xen-defaults.bbclass
+++ b/classes/qemuboot-xen-defaults.bbclass
@@ -23,8 +23,10 @@ QB_DEFAULT_KERNEL:qemuarm = "xen-${MACHINE}"
 
 # 64-bit Arm: gic version 3
 QB_MACHINE:qemuarm64 = "-machine virt,gic-version=3 -machine virtualization=true"
-# 32-bit Arm
-QB_MACHINE:qemuarm = "-machine virt -machine virtualization=true"
+# 32-bit Arm: highmem=off
+# Disable highmem so that qemu does not use highmem IO regions that end up
+# being placed at the 256GiB mark (e.g. ECAM space) and can cause issues in Xen.
+QB_MACHINE:qemuarm = "-machine virt,highmem=off -machine virtualization=true"
 
 # Increase the default qemu memory allocation to allow for the hypervisor.
 # Use a weak assignment to allow for change of default and override elsewhere.