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

[Andre Przywara]

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.
Yes, it was enabled in the qemu64 type since "the dawn of time". But at 
the same moment it was unconditionally (both for TCG and KVM) masked out 
later, so there were no 3DNOW bits at any time. Commit 671e4676 changed 
that to be only applicable to TCG. So in the last three weeks or so by 
default there were 3DNOW bits in KVM on AMD processors.
This is a particular problem with DirectX9 (WindowsXP), which still uses 
3DNow if available.

> 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.
Well, I'd like to leave TCG out of the migration theme by default. If 
you seriously use KVM and migration, I suppose TCG is not an option for 
you. It's a nice thing to have, but for different things than KVM.
But I will check how much difference there is between Prescott-2M, 
K8RevF and TCG, maybe we can leave it in.

Regards,
Andre.

> That would also make it easier to know where to put other fancy features 
> like "SVM" :-).
>
> Alex


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Andre Przywara
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