Re: [PATCH] kvm: Set default accelerator to "kvm" if the host supports it

[Jan Kiszka <jan.kiszka@web.de>]

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On 2012-10-03 08:58, Michael Tokarev wrote:
> On 02.10.2012 11:46, Markus Armbruster wrote:
>> "Daniel P. Berrange" <berrange@redhat.com> writes:
> 
>>> IMHO, default to KVM, fallback to TCG is the most friendly default
>>> behaviour.
>>
>> Friendly perhaps, generating an infinite series of questions "why is my
>> guest slow as molasses?" certainly.
> 
> With a warning about "switching to slow emulation mode because .."
> printed at startup that becomes a non-issue, because there's no
> reason to ask more questions about why it is slow - it already
> said why.  Yes some may try to ask what to do, which is different.
> 
> Every howto nowadays mentions kvm modules and /dev/kvm device
> permissions.
> 
>> And for each instance of the question, there's an unknown number of
>> users who give QEMU a quick try, screw up KVM unknowingly, observe the
>> glacial speed, and conclude it's crap.
> 
> This is, again, I think, unfair.  With the warning message it becomes
> more or less obvious.
> 
> If you're talking about users who run it with -daemonize argument -
> this is a) stupid to do when TRYING it out, so it's not a big deal
> to lose another stupid user, and b) qemu should init everything
> first and throw all warnings and fatal errors before daemonizing,
> if this is not the case it should be fixed in the code.
> 
> And if you're talking about management software (libvirt and others),
> it controls all the required privileges already and explicitly
> requests acceleration and other stuff.
> 
> So the best thing to do is what Daniel, Aurelien, Paolo and others
> are suggested: accel=kvm:tcg with a warning.

Well, we had a lot of problems with such a fallback in the past, but I
think we had no proper warnings back then.

I'm not fully believing in users will always realize the console
message. I would therefore suggest to change the window title of QEMU as
well if we fail to initialize some accelerator. Something like "QEMU
without KVM" (could be "QEMU without $FAILED_ACCEL" in the end, i.e. not
just for KVM). If that makes sense for everyone, I'll hack the required
patches.

Jan



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