Re: [Qemu-devel] [PATCH] omit 3DNOW! CPUID bits from qemu64 CPU model

[Alexander Graf]

On 16.07.2009, at 18:01, Jamie Lokier wrote:

> Alexander Graf wrote:
> > On 16.07.2009, at 14:49, Andre Przywara wrote:
> >
> > > Since we recently do not disable 3DNOW! support anymore, we should
> > > avoid setting the bits in the default qemu64 CPU model to ease
> > > migration. TCG does not support it anyway and even AMD deprecates
> > > it's usage nowadays.
> >
> > TCG does not implement it but it was enabled in the qemu64 type? That
> > sounds like a serious bug people would have found before.
>
> Almost nobody uses 3DNow! with a 64-bit guest, so it's not so
> easy to notice.
>
> But I've have expected people running 32-bit guests on a qemu64 CPU to
> notice, even if it's just the prefetch instructions...  unless those
> happen to overlap with NOPs on non-3DNow! hardware.  (I'm too lazy  
> to check).
>
> > I really think we should try and keep the "qemu64" type (TCG
> > capabilities) and the "kvm safe" type separate. IMHO the best  
> > scenario
> > would be a -cpu "safe" type, used as default, that is the common
> > dominator between KVM on VMX, KVM on SVM and TCG.
>
> qemu32 => TCG 32-bit
> qemu64 => TCG 64-bit
>
> safe32 => common between KVM and TCG, 32-bit
> safe64 => common between KVM and TCG, 64-bit

Yeah. Maybe only "safe" and always assume x86_64? But then comes  
Intel ... sigh.

> > That would also make it easier to know where to put other fancy
> > features like "SVM" :-)
>
> Can't we use SVM emulation with TCG and KVM both?

We can use SVM emulation with TCG and KVM on SVM. We can't on KVM with  
VMX and Xen HVM. For KVM it's fine though, because the feature gets  
masked out if it's not supported.

Still, SVM isn't migratable on KVM atm, so let's better disable it for  
"safe".

Alex