Re: [PATCH] multiboot: Use DMA instead port-based transfer

[Stefano Garzarella]

CCing Paolo since kvm-unit-tests use multiboot.

On Sun, Oct 10, 2021 at 09:10:05PM +0200, Adam Lackorzynski wrote:
> Use DMA transfers in the multiboot loader to copy
> data.
>
> This significantly lowers QEMU's startup latency by
> a factor of about 40, for example, going from 30sec
> to 0.8sec when loading modules of 120MB in size.
> This change has been used successfully for some time.
>
> Signed-off-by: Marcus Hähnel <marcus.haehnel@kernkonzept.com>
> Signed-off-by: Adam Lackorzynski <adam@l4re.org>
> ---
> pc-bios/multiboot.bin         | Bin 1024 -> 1536 bytes
> pc-bios/optionrom/multiboot.S |  10 ++---
> pc-bios/optionrom/optionrom.h |  77 ++++++++++++++++++++++++++++++++++
> 3 files changed, 82 insertions(+), 5 deletions(-)

I tested with https://gitlab.com/kvm-unit-tests/kvm-unit-tests running:
 export QEMU=`/path/to/qemu-system-x86_64 -L path/to/pc-bios/optionrom`
 time ./run_test.sh

Before this patch:
real    3m5.578s
user    2m32.231s
sys     0m50.390s

After this patch:
real    2m55.614s
user    2m20.536s
sys     0m50.046s

It's still a good improvement, considering the fact that the images 
aren't big.

Just a consideration, for blobs (kernel and initrd) I fully agree to use 
DMA, but for 32bit variables is it useful?

However, this patch LGTM:

Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>