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

[Paolo Bonzini]

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.

Paolo