Re: [Qemu-devel] [RFC PATCH v2 00/12] Guest startup time optimization

[Paolo Bonzini]

On 25/08/2016 12:14, Chao Peng wrote:
> This patchset is trying to optimize guest startup time by disabling
> or simplifying some features in QEMU. The version 1 can be found at:
> https://lists.nongnu.org/archive/html/qemu-devel/2016-06/msg04842.html
> 
> Unlike version 1, this version optimizes Q35 directly instead of
> introducing a totally new platform. But we still keep the design of
> skipping firmware as till now we don't have good idea to optimize
> firmware to get the comparable boot time.
> 
> The patchset is against commit 5f0e775 (Update version for v2.7.0-rc3
> release) on master branch.
> 
> Basically this patchset introduces several switches to qemu comandline
> so that several features can be turned off in some use cases. The
> default behavior will not change in case no switches are provided.
> 
> Performance data:
> feature(switches)       time saved in guest
> -nosmbus                4ms
> -nosata                 6ms
> -nopic                  2ms
> -nopit                  5ms
> -static-prt             8ms
> -nofw                   62ms
> 
> Thanks,
> Chao
> 
> Chao Peng (9):
>   pc: make smbus configurable
>   pc: make sata configurable
>   pc: make pic configurable
>   pc: make pit configurable
>   acpi: build static _PRT
>   ich9: enable pm registers when there is no firmware
>   q35: initialize MMCFG base when there is no firmware
>   pc: support direct loading protected/long mode kernel
>   pc: skip firmware
> 
> Haozhong Zhang (3):
>   acpi: expose data structurs and functions of BIOS linker loader
>   acpi: expose acpi_checksum()
>   acpi: patch guest ACPI when there is no firmware
> 
>  hw/acpi/bios-linker-loader.c         |  83 +---------
>  hw/acpi/core.c                       |   2 +-
>  hw/acpi/nvdimm.c                     |   6 +-
>  hw/i386/Makefile.objs                |   2 +-
>  hw/i386/acpi-build-nofw.c            | 295 ++++++++++++++++++++++++++++++++++
>  hw/i386/acpi-build.c                 | 102 +++++++-----
>  hw/i386/acpi-build.h                 |   5 +
>  hw/i386/pc.c                         | 301 
> +++++++++++++++++++++++++++++++----
>  hw/i386/pc_piix.c                    |   2 +-
>  hw/i386/pc_q35.c                     |  60 ++++---
>  hw/isa/lpc_ich9.c                    |  12 +-
>  hw/pci-host/q35.c                    |  15 +-
>  include/hw/acpi/acpi.h               |   2 +
>  include/hw/acpi/bios-linker-loader.h |  85 ++++++++++
>  include/hw/i386/pc.h                 |  16 +-
>  15 files changed, 800 insertions(+), 188 deletions(-)
>  create mode 100644 hw/i386/acpi-build-nofw.c

Patches 1-4 are okay, though I think it would be easier to add a -M
q35-lite too that just removes the legacy devices.  The -M q35-lite
machine doesn't have to support versioning for now.

As you might expect, I don't agree with removing the firmware.  There's
room for much more optimization before duplicating firmware code in
QEMU.  I'd rather see numbers for:

1) qboot optimizations: adopt the fw_cfg DMA interface instead of the
cbfs flash hack (so that -kernel works), drop PCI bridge initialization,
copy less than 64K of memory from ROM to 0xf0000;

2) Linux optimizations: using an uncompressed image to avoid the cost of
copying and decompressing.  QEMU can already load the image at the right
place and the real mode stub can do little more than GDT/IDT setup.

3) PAM optimizations: for -M q35-lite initialize the machine with RAM
from 0xc0000 to 1MB.

I know that you ultimately would like to mmap the kernel, but I would
like to have a better understanding of where the time is spent.

Thanks,

Paolo