On Thu, 14 Feb 2013 11:51:59 -0200 Eduardo Habkost <ehabk...@redhat.com> wrote:
> 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? Problem was that gcc generated 2 LOAD sections with a hole ~128Mb between them. And current implementation of ELF multiboot loader in QEMU just allocates flat buffer of size=highest-lowest loaded address from ELF which includes possible holes. Possible fix is to exclude holes and pass a packed buffer to seabios, which would require fixing multiboot rom as well (not sure if it could be made compatible with migration to old qemu/seabios). this hole.
