Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host

[Paul Brook <paul@codesourcery.com>]

> KVM users wants performance in their apps anything else is secondary,
> otherwise they would be fine using qemu, no?

No. According to others in this thread, migration between different hosts is 
also important, as is isolation from host machine specifics. 

In fact I'm skeptical how much benefit using an Intel/AMD vendor ID gets you 
by way of performance. While you may be running code "natively", there's still 
an awful lot of things that trap back to the hypervisor, so you're liable to 
get different performance characteristics to a native CPU.

> Besides I think there is breakage in setting the SEP feature on athlon
> definition for example, I don't have an hardware athlon around to
> check /proc/cpuinfo though, but I don't think SEP would be enabled
> there.
>
> Let's focus on performance (and facts like SEP being enabled on
> athlon) without shooting ourself in the foot by breaking all guest OS
> assumptions with an unknown qemu vendor ID.
>
> Or if you really want to argue, in most others thread you pretend qemu
> should be as close as possible as hardware (no matter if guest
> breaks), and I know no hardware with your brand new qemu vendor_id, do
> you? (yeah not even the model name is reflecting hardware very well I
> know, but that didn't happen to break stuff yet so it can stay as
> default for now)

You're missing the point. "-cpu host" or "-cpu p6" (where p6 is lowest-common-
denominator) may be a reasonable default for KVM. What's not acceptable (as 
evidenced by this bug) is taking an arbitrary CPUID and blindly sticking an 
Intel vendor ID on it.

Paul