Re: [PATCH] qemu-kvm: Switch to upstream -enable-kvm semantics

[Markus Armbruster <armbru@redhat.com>]

Anthony Liguori <anthony@codemonkey.ws> writes:

> On 12/15/2010 09:50 AM, Markus Armbruster wrote:
>> We currently enable KVM by default, and when it's not available, we
>> print a message and fall back to TCG.  Option -enable-kvm is ignored.
>> Option -no-kvm suppresses KVM.
>>
>> Upstream works differently: KVM is off by default, -enable-kvm
>> switches it on.  -enable-kvm terminates the process unsuccessfully if
>> KVM is not available.
>>
>> upstream qemu   |  default  |-enable-kvm
>> ----------------+-----------+-----------
>> KVM available   | disabled  |  enabled
>> KVM unavailable | disabled  |    fail
>>
>> qemu-kvm        |  default  |-enable-kvm
>> ----------------+-----------+-----------
>> KVM available   |  enabled* |  enabled
>> KVM unavailable | disabled  | disabled*
>>
>> * differs from upstream
>>
>> Users of qemu and qemu-kvm need to be aware of these differences to
>> enable / disable use of KVM reliably.  This is bothersome.
>>
>> Consider -enable-kvm when KVM is unavailable: If the user expects
>> qemu-kvm behavior (fall back), but qemu fails, he'll likely be
>> surprised and unhappy.  If the user expects upstream behavior (fail),
>> but qemu-kvm falls back to TCG, the guest runs slow as molasses, and
>> the user will likely be confused and unhappy (unless he spots and
>> understands the "disable KVM" message).
>>
>> Switch to upstream semantics: KVM off by default, -enable-kvm switches
>> it on, and when it can't, it's fatal.
>>
>> Having to enable KVM explicitly is annoying, but the proper place to
>> address that is upstream.
>>
>> Signed-off-by: Markus Armbruster<armbru@redhat.com>
>>    
>
> Backwards compatibility is going to kill us if we try to make this change.
>
> Current qemu-kvm behavior:
>
> default: -accel kvm,tcg
> -no-kvm: -accel tcg
> -enable-kvm: -accel kvm,tcg
>
> Current upstream behavior
>
> default: -accel tcg
> -enable-kvm: -accel kvm
>
> I think we should tie `-accel' to the machine type.  For qemu-kvm, a
> different default machine type should be used than upstream qemu (it
> really should be a configure switch).
>
> For `pc', the default `-accel' behavior should remain 'tcg'.  For
> kvmpc', the default `-accel' behavior should be 'kvm,tcg'.
>
> -no-kvm should be deprecated.  -enable-kvm should also be deprecated
> in favor of the `-accel' option.

I'm fine with -accel and deprecating the old options.  But until we have
that:

> In the short term, it would be a good idea to modify qemu-kvm to
> switch the -enable-kvm semantics to match upstream (fail if KVM isn't
> available).

That's what my patch does.

Additionally, it changes the default to match upstream: KVM disabled.

What do you want changed in my patch?

>              Adding an alias for 'kvmpc' upstream and qemu-kvm and
> making qemu-kvm default to 'kvmpc' would be helpful for management
> tools too.

That would address "Having to enable KVM explicitly is annoying".