Re: [Qemu-devel] [PATCH] qemu:cpuid: speedup test by 3x times if grub2 is available

[Eduardo Habkost]

On Thu, Feb 14, 2013 at 02:31:38PM +0100, Paolo Bonzini wrote:
> Il 14/02/2013 14:24, Eduardo Habkost ha scritto:
> > On Thu, Feb 14, 2013 at 01:13:18PM +0100, Paolo Bonzini wrote:
> >> Il 14/02/2013 12:18, Eduardo Habkost ha scritto:
> >>>>> qemu boots from disk image 3 times faster than direct kernel load.
> >>> That's surprising. Do you have any idea why that happens?
> >>
> >> Because kernel load uses MMIO (from fw_cfg), while booting from disk
> >> uses at worst PCI DMA and at best virtio.
> > 
> > Is it something worth trying to optimize
> 
> I think that, within the limits of what the spec makes legal, Gleb
> optimized all that he could out of it.  The alternative is to make
> fw_cfg do DMA, which in the past was rejected because it doesn't look
> like what real ISA hardware would do.
> 
> > , or a reasonable solution would
> > be so similar to having a disk+bootloader that's easier to simply
> > recommend people to set up a real disk with a real bootloader if they
> > care about speed?
> 
> In the end it's a pity, but yeah that's the easiest thing to do with
> distro kernels and big all-drivers initrd.  -kernel is still useful and
> fast enough if you have a custom-built kernel, possibly with no initrd
> at all.

The patch that originated this thread wasn't even for distro kernels and
big initrds. Our test case lods a very small test kernel (11 KB), and it
is taking almost 15 seconds to boot.

Maybe our test case should create a custom BIOS image to be loaded on
ROM, instead?

-- 
Eduardo