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Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2

From: Gleb Natapov
Subject: Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?
Date: Tue, 3 Aug 2010 22:26:13 +0300

On Tue, Aug 03, 2010 at 02:15:05PM -0500, Anthony Liguori wrote:
> On 08/03/2010 02:05 PM, Gleb Natapov wrote:
> >On Tue, Aug 03, 2010 at 09:43:39PM +0300, Avi Kivity wrote:
> >>>If Richard is willing to do the work to make -kernel perform
> >>>faster in such a way that it fits into the overall mission of what
> >>>we're building, then I see no reason to reject it.  The criteria
> >>>for evaluating a patch should only depend on how it affects other
> >>>areas of qemu and whether it impacts overall usability.
> >>That's true, but extending fwcfg doesn't fit into the overall
> >>picture well.  We have well defined interfaces for pushing data into
> >>a guest: virtio-serial (dma upload), virtio-blk (adds demand
> >>paging), and virtio-p9fs (no image needed).  Adapting libguestfs to
> >>use one of these is a better move than adding yet another interface.
> >>
> >+1. I already proposed that. Nobody objects against fast fast
> >communication channel between guest and host. In fact we have one:
> >virtio-serial. Of course it is much easier to hack dma semantic into
> >fw_cfg interface than add virtio-serial to seabios, but it doesn't make
> >it right. Does virtio-serial has to be exposed as PCI to a guest or can
> >we expose it as ISA device too in case someone want to use -kernel option
> >but do not see additional PCI device in a guest?
> fw_cfg has to be available pretty early on so relying on a PCI
> device isn't reasonable.  Having dual interfaces seems wasteful.
fw_cfg wasn't mean to be used for bulk transfers (seabios doesn't even
use string pio to access it which make load time 50 times slower that
what Richard reports). It was meant to be easy to use on very early
stages of booting. Kernel/initrd are loaded on very late stage of
booting at which point PCI is fully initialized.

> We're already doing bulk data transfer over fw_cfg as we need to do
> it to transfer roms and potentially a boot splash.  Even outside of
> loading an initrd, the performance is going to start to matter with
> a large number of devices.
Most roms are loaded from rom PIC bars, so this leaves us with boot
splash, but boot splash image should be relatively small and if user
wants it he does not care about boot time already since bios need to
pause to show the boot splash anyway.


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