Re: [Qemu-devel] [PATCH v4 0/3] pc: mmap kernel (ELF image) and initrd

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefano Garzarella (sgarzare@redhat.com) wrote:
> 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.
> 
> v4:
>   - Patch 1: fix the rom_add_elf_program() comment [Paolo]
>   - Patch 2:
>     ~ fix the missing of g_mapped_file_unref() in the success case [Paolo]
>     ~ fix the rom_add_elf_program() comment [Paolo]
> 
> v3: https://patchew.org/QEMU/20190724112531.232260-1-sgarzare@redhat.com/
> v2: https://patchew.org/QEMU/20190723140445.12748-1-sgarzare@redhat.com/

Two high level questions:
   a) What happens if someone tries to migrate the VM - I don't think
it's too unusual for people to run with -kernel/-initrd in situations
where they migrate.

   b) Are there situations where you can't mmap but you can validly
read it?  For example, running with an ELF built for 4k page alignment
on a host with 64k host pages?

Dave

> 
> 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 (3):
>   loader: Handle memory-mapped ELFs
>   elf-ops.h: Map into memory the ELF to load
>   hw/i386/pc: Map into memory the initrd
> 
>  hw/core/loader.c     | 38 +++++++++++++++++++-----
>  hw/i386/pc.c         | 17 ++++++++---
>  include/hw/elf_ops.h | 71 ++++++++++++++++++++++++++------------------
>  include/hw/i386/pc.h |  1 +
>  include/hw/loader.h  |  5 ++--
>  5 files changed, 89 insertions(+), 43 deletions(-)
> 
> -- 
> 2.20.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK