[Qemu-devel] [PATCH v2 0/2] pc: mmap kernel (ELF image) and initrd

[Stefano Garzarella <sgarzare@redhat.com>]

In order to reduce the memory footprint when PVH kernel and initrd
are used, we map them into memory instead of reading them.
In this way we can share them between multiple instances of QEMU.

v2:
- Patch 1: used g_mapped_file_new_from_fd() with 'writeble' set to 'true',
           since we can modify the mapped buffer. [Paolo, Peter]

These are the results using a PVH kernel and initrd (cpio):
- memory footprint (using smem) [MB]
        QEMU              before                   now
    # instances        USS      PSS            USS      PSS
         1           102.0M   105.8M         102.3M   106.2M
         2            94.6M   101.2M          72.3M    90.1M
         4            94.1M    98.0M          72.0M    81.5M
         8            94.0M    96.2M          71.8M    76.9M
        16            93.9M    95.1M          71.6M    74.3M

    Initrd size: 3.0M
    Kernel
        image size: 28M
        sections size [size -A -d vmlinux]:  18.9M

- boot time [ms]
                          before                   now
 qemu_init_end:           63.85                   55.91
 linux_start_kernel:      82.11 (+18.26)          74.51 (+18.60)
 linux_start_user:       169.94 (+87.83)         159.06 (+84.56)

QEMU command used:
./qemu-system-x86_64 -bios /path/to/seabios/out/bios.bin -no-hpet \
    -machine q35,accel=kvm,kernel_irqchip,nvdimm,sata=off,smbus=off,vmport=off \
    -cpu host -m 1G -smp 1 -vga none -display none -no-user-config -nodefaults \
    -kernel /path/to/vmlinux -initrd /path/to/rootfs.cpio \
    -append 'root=/dev/mem0 ro console=hvc0 pci=lastbus=0 nosmap'

Stefano Garzarella (2):
  elf-ops.h: Map into memory the ELF to load
  hw/i386/pc: Map into memory the initrd

 hw/i386/pc.c         | 15 ++++++++---
 include/hw/elf_ops.h | 64 ++++++++++++++++++++++++--------------------
 2 files changed, 46 insertions(+), 33 deletions(-)

-- 
2.20.1