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

[Alexander Graf]

On 19.07.2010, at 10:19, Gleb Natapov wrote:

> 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?

The kernel and initrd are read directly from the host fs. It's more like a 9p 
grub boot.

> 
>>> 
>>>>> 
>>>>>> 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?

Yes and no. It sounds nice at first, but doesn't quite fit. There are two 
issues:

1) We need a new PCI ID
2) There can be a lot of initrd binaries with multiboot. We only have a limited 
amount of BARs


Alex