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
