Cannot launch with 1GB hugepages (QEMU 5.0)

[Adam Gradzki]

Hello,

I have a question about how QEMU uses hugepages.

Hugepages are 2048 kB by default on Arch Linux (Linux 5.6.15-arch1-1 #1 SMP 
PREEMPT Wed, 27 May 2020 23:42:26 +0000 GNU/Linux)

I do not encounter any QEMU problems when using this default configuration. I 
create roughly 8GB of hugepages with this command:
sudo su -c "echo 4000 > /proc/sys/vm/nr_hugepages"

I am then able to pass "-mem-path /dev/hugepages" to qemu-system-x86_64 and my 
VM launches successfully.

However, when I set hugepages to 1 GB and create eight of them with this 
command:
sudo su -c "echo 8 > /proc/sys/vm/nr_hugepages"

QEMU stops immediately with an error: qemu-system-x86_64:  : drive with bus=0, 
unit=0 (index=0) exists

As an experiment I tried mounting a hugetlbfs on /dev/hugepages:
sudo mount -t hugetlbfs -o mode=01770,gid=1000 hugetlbfs /dev/hugepages

I confirmed that the mount worked with "mount | grep huge":
cgroup on /sys/fs/cgroup/hugetlb type cgroup 
(rw,nosuid,nodev,noexec,relatime,hugetlb)
hugetlbfs on /dev/hugepages type hugetlbfs 
(rw,relatime,gid=1000,mode=1770,pagesize=1024M)

/proc/cmdline:
root=/dev/nvme0n1p2 rw initrd=\initramfs-linux.img i915.enable_gvt=1 
intel_iommu=igfx_off kvm.ignore_msrs=1 kvm.report_ignored_msrs=0 
default_hugepagesz=1G

The QEMU error is the same as before. What can I do to get 1 GB hugepages 
working with QEMU 5.0?

-- 
Adam Gradzki