Re: [PATCH] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

[Ingo Molnar]

* Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

> > Hmm.
> > 
> > If we did this, I think I'd prefer a slightly more general approach. First 
> > teach KVM to support a mode in which it's purely an emulator (Paolo: how hard 
> > is this?  It would also make testing the emulator much easier).
> 
> This isn't hard, at least for Intel: make emulation_required() return true 
> always (and fix the fallout).  However, it's not necessary.  The emulator is 
> designed to be independent from the rest of KVM.  At some point I think Avi was 
> testing it in userspace (or planning to do so). So you would just move it from 
> arch/x86/kvm to arch/x86/emulate.

Very nice!

> The obvious downside is that the emulator isn't really designed for speed.
>
> In KVM it's currently 1000-1500 times slower than the real thing.  Even if you 
> modified it to remove the KVM overhead (vm86 is just running ring 3 code; no 
> interrupts and no pagetables to walk), it probably would take 300-500 cycles to 
> execute one instruction.

This needs to be tested, but I wouldn't expect it to be a big issue:

 - if anyone cares they can improve its performance

 - or worst case they can upgrade their tool to something newer which will use 
   user-space emulation of 16-bit code anyway ...

 - Furthermore I suspect with vm86 we'd trap out of vm86 mode rather often - and a 
   single trap can take thousands of cycles. So I suspect the effective slowdown 
   depends on the workload.

 - In the absolute worst case it will perform like a really old CPU.

Thanks,

	Ingo
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