Re: [Qemu-devel] Question about qemu firmware configuration (fw_cfg) device

[Gleb Natapov]

On Mon, Jul 19, 2010 at 10:08:57AM +0200, Alexander Graf wrote:
> 
> On 19.07.2010, at 10:01, Gleb Natapov wrote:
> 
> > On Mon, Jul 19, 2010 at 09:57:02AM +0200, Alexander Graf wrote:
> >> 
> >> On 19.07.2010, at 09:51, Gleb Natapov wrote:
> >> 
> >>> On Mon, Jul 19, 2010 at 09:40:18AM +0200, Alexander Graf wrote:
> >>>> 
> >>>> On 19.07.2010, at 09:33, Gleb Natapov wrote:
> >>>> 
> >>>>> On Mon, Jul 19, 2010 at 08:28:02AM +0100, Richard W.M. Jones wrote:
> >>>>>> On Mon, Jul 19, 2010 at 09:23:56AM +0300, Gleb Natapov wrote:
> >>>>>>> That what I am warring about too. If we are adding device we have to 
> >>>>>>> be
> >>>>>>> sure such device can actually exist on real hw too otherwise we may 
> >>>>>>> have
> >>>>>>> problems later.
> >>>>>> 
> >>>>>> I don't understand why the constraints of real h/w have anything to do
> >>>>>> with this.  Can you explain?
> >>>>>> 
> >>>>> Each time we do something not architectural it cause us troubles later.
> >>>>> So constraints of real h/w is our constrains to.
> >>>>> 
> >>>>>>> Also 1 second on 100M file does not look like huge gain to me.
> >>>>>> 
> >>>>>> Every second counts.  We're trying to get libguestfs boot times down
> >>>>>> from 8-12 seconds to 4-5 seconds.  For many cases it's an interactive
> >>>>>> program.
> >>>>>> 
> >>>>> So what about making initrd smaller? I remember managing two
> >>>>> distribution in 64M flash in embedded project.
> >>>> 
> >>>> Having a huge initrd basically helps in reusing a lot of existing code. 
> >>>> We do the same - in general the initrd is just a subset of the 
> >>>> applications of the host OS. And if you start putting perl or the likes 
> >>>> into it, it becomes big.
> >>>> 
> >>> Why not provide small disk/cdrom with all those utilities installed?
> >> 
> >> Because - if the loading is done fast - this way everything's in RAM 
> >> instantly. And you still have all devices available for use inside the 
> >> system - that makes enumeration a lot easier. There are several reasons 
> >> why and I don't think we should force different ways on people just 
> >> because one component of our system is ineffective.
> >> 
> > Loading huge initrd on real HW takes noticeably longer time that small
> > one, so I would say that it is your design that is to blame here, not
> > KVM.
> 
> I disagree. Virtualization enables new use cases. The -initrd parameter is a 
> very good example for that. It's something that you simply couldn't do on 
> real hw.
> 
How is it different from starting kernel/initrd from usb flash drive?

> > 
> >>> 
> >>>> I guess the best thing for now really is to try and see which code paths 
> >>>> insb goes along. It should really be coalesced.
> >>>> 
> >>> It is coalesced to a certain extent (reenter guest every 1024 bytes,
> >>> read from userspace page at a time). You need to continue injecting
> >>> interrupt into a guest during long string operation and checking
> >>> exception condition on a page boundaries.
> >> 
> >> That still sounds slow. So yeah, adding DMA is probably the right way to 
> >> go. But then again - if we model it after real hw it would be 
> >> asynchronous, giving us an interrupt, causing even more headache. Ugh.
> >> 
> >> Can't we just ignore real hw constraints here and have it available in 
> >> guest ram once one particular PIO is done? No bus master, no interrupts, 
> >> but full speed and simplicity/atomicity which also helps migration.
> >> 
> > We shouldn't add devices that work not like real HW to speed up some
> > pathological cases (and are slow on real HW too).
> 
> Just because you don't use them doesn't mean they're pathological, really. We 
> simply chose a bad interface for transferring reasonable big chunks of data 
> and we need to fix that. If you want to look at it from a different 
> perspective, it's a regression. Older qemu versions did map the kernel and 
> initrd directly into guest ram, so now we're slower than back then.
> 
I use them hundred time each day (at least -kernel part). If the
interface is slow for your use case I have no problem with introducing
new one, but the one that make sense in x86 architecture. I do not agree
this is regression BTW. You can't compare buggy way of doing things and
non-buggy way and say that bug fixing is a regression.

What about adding new PCI card that holds kernel initrd in ROM bar?

--
                        Gleb.