Re: KVM, AMD & Interrupts

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Matthew Hall wrote:
> Hi Avi,
>
> I spoke to you a while ago on #kvm on freenode - I had a problem with 
> ide ops on an amd svm host with kvm. The issues regarded the inability 
> to access ida devices (hda/hdc) under qemu & kvm on my host - if this 
> triggers your memory...
>
> The last point I got to was that it was kvm and amd specific. Id est, 
> the same image run under an intel vmx host with the *exact* same 
> image, kvm version and command options (svn 'trunk' from the $date).
> The issue seemed to be AMD and kvm specific (running with '-no-kvm' 
> made everything work ok, but slower, as you'd expect).
>
> I got word of a possible workaround, from one of your colleagues 
> whilst you've been away, to use the boot options 'noapic nolapic' on 
> the host o/s. Using these options solved my hd* op problems and I can 
> boot ok now.
> It seems disabling the local apic was the fundamental part (iirc), as 
> without host support the guest would always start the dummy apic; so 
> running the guest with any 'special' options (ie. '-no-apic') should 
> not be required.
>
> I'm think this problem is specific to my particular cpu/motherboard 
> combination aswell.
> I also have box with a recent intel cpu (with the vmx extention) which 
> doesn't display this behaviour; and from talking with you it's not 
> purely amd specific (I believe your tests on amd cpu worked).
> Is it possible a slowdown in the interrupt controller between the 
> guest and host is a contributing factor?
> Iirc, with a 'dummy' apic, I experience a ~5% slowdown on disk and 
> network operations, which isn't so bad. I've yet to think about smp 
> usage.

I'll have to refresh my memory of the AMD spec (I didn't do the AMD port 
so the details are fuzzy).  Perhaps the AMD folks have some insight.  
Maybe it's just another missing event to intercept.

>
> Thought you might appreciate this info should you feel it deserve a 
> wiki entry. Point me in the direction of the appropriate page should 
> you feel I can put this info in myself.
>
>

The best place for such things is the bug tracker (you can find a link 
on the wiki).


-- 
error compiling committee.c: too many arguments to function


-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys - and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV