Question about the kvm emulator

Question about the kvm emulator

[Jidong Xiao]

Hi,

I noticed that there is a file call emulate.c, under the directory of
arch/x86/kvm/, in its header part, it says:

"Generic x86 (32-bit and 64-bit) instruction decoder and emulator."

I am confused about this, since qemu will be the emulator, why does
kvm itself also includes such an emulator? In particular, I added some
printk debug statement in x86_emulate_insn() and
emulate_instruction(), but I never see them being invoked. Can someone
kindly explain this, thank you!

-Jidong

Re: Question about the kvm emulator

[Paolo Bonzini]

Il 22/05/2014 05:55, Jidong Xiao ha scritto:
> Hi,
>
> I noticed that there is a file call emulate.c, under the directory of
> arch/x86/kvm/, in its header part, it says:
>
> "Generic x86 (32-bit and 64-bit) instruction decoder and emulator."
>
> I am confused about this, since qemu will be the emulator, why does
> kvm itself also includes such an emulator? In particular, I added some
> printk debug statement in x86_emulate_insn() and
> emulate_instruction(), but I never see them being invoked. Can someone
> kindly explain this, thank you!

QEMU is providing the device model and interfaces with KVM.  QEMU's CPU 
emulation functionality is not used when running with KVM.

The KVM emulator is used: 1) to execute instructions that read or write 
to device memory; 2) to run "big real mode" programs on microprocessors 
that do not support it natively, for example Intel processors older than 
Westmere.

Paolo

Re: Question about the kvm emulator

[Jidong Xiao]

Thanks Paolo, it's very clear.

-Jidong

On Thu, May 22, 2014 at 7:42 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> Il 22/05/2014 05:55, Jidong Xiao ha scritto:
>
>> Hi,
>>
>> I noticed that there is a file call emulate.c, under the directory of
>> arch/x86/kvm/, in its header part, it says:
>>
>> "Generic x86 (32-bit and 64-bit) instruction decoder and emulator."
>>
>> I am confused about this, since qemu will be the emulator, why does
>> kvm itself also includes such an emulator? In particular, I added some
>> printk debug statement in x86_emulate_insn() and
>> emulate_instruction(), but I never see them being invoked. Can someone
>> kindly explain this, thank you!
>
>
> QEMU is providing the device model and interfaces with KVM.  QEMU's CPU
> emulation functionality is not used when running with KVM.
>
> The KVM emulator is used: 1) to execute instructions that read or write to
> device memory; 2) to run "big real mode" programs on microprocessors that do
> not support it natively, for example Intel processors older than Westmere.
>
> Paolo
>