Re: [kvm-devel] [Qemu-devel] expose host CPU features to guests: Take 3

[Avi Kivity <avi@qumranet.com>]

Jamie Lokier wrote:
> Jocelyn Mayer wrote:
>   
>> Well, it may be needed to integrate the "-cpu host" option.
>> But I think it's a really bad idea that running qemu without the -cpu
>> option would not act the same on different host. When I want to test
>> qemu with the same command line, I want it to behave exactly the same,
>> whatever the host I'm running on, like any other tool. Think about gcc,
>> for example, if you launch it without option, it compiles for a generic
>> CPU. If you want to tune the generated code, even for the host you're
>> running on, you have to use -mcpu/-march/-mtune. Using one command line
>> always have gives the same result.
>> Then, my opinion is that running qemu without any -cpu option should
>> always use a default target.
>>     
>
> A major feature of qemu is reproducible behaviour.  This is especially
> important when resuming snapshots or booting pre-installed images for
> testing things.
>
> A major feature of kvm/kqemu is performance.  But not always
> performance at the expense of reproducibility.  Sometimes you want to
> use kvm/kqemu to make qemu as fast as possible while still behaving
> the same with some image.
>
> Where someone wants performance to have precedence, it's easy enough
> to advise "use -cpu host" for that.
>
> Where someone wants performance, but reproducibility to have
> precedence, it would be more awkward to have to advise "use -cpu
> list,of,common,features".
>   

Or maybe "-cpu baseline" or some other neutral word.

> I think reproducibility is something which needs to be working by
> default.  This is because people make images before they decide to
> move them to other hosts, and often before they would consider at all
> issues about host differences, and making images is sometimes a lot of
> work.  Images should be portable among hosts by default.
>
> But, if we say that the default is a baseline CPU, and there's a "-cpu
> host" option to maximise performance, then we have to decide what
> features the baseline CPU has.  Obviously a 386 isn't very useful for
> emulation any more - many OSes won't run on it.
>
>   

The baseline cpu should be whatever qemu supports today.

> Then, if we change our mind about the default baseline CPU, then we've
> lost reproducibility for old images anyway.
>
> That's an argument for storing the emulated CPU features in images.
>   

That's another thread -- I want memory size and network configuration in 
there too :)

> If that's done, then for image portability among hosts, it is not so
> important that the default CPU have only the features which
> are native to every host.  Instead, the critical thing is simply that
> it has only features which are either native or can be emulated on
> every host.  E.g. TSC is in this category.
>   

Qemu is used to support three different user classes which have 
different requirements. Developers want reproducibility and control, so 
-cpu baseline should be the default for them. Home users want speed to 
run Windows or test distros, so -cpu host and kvm should be the default 
for them. Managed deployments want something in between, so they'd 
specify -cpu feature,feature and won't care about the default.

We can make it a ./configure option. I suspect distros will want to set 
-cpu host as default as their users primarily care about speed.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.