Patchwork Re: [questions] savevm|loadvm

Does current qemu-kvm (qemu v0.12.3) use the irqchip, pit of KVM? I
cannot find any KVM_CREATE_IRQCHIP and KVM_CREATE_PIT in the qemu
code.

Concerning the interface between qemu and kvm, I have the following confusion:
1. How irqchip and pit of KVM collaborating with the irq and pit
emulation of QEMU? As far as I see, qemu-kvm still uses qemu's irq and
pit emulation, doesn't it?

2. For return from KVM to QEMU, I cannot get the meaning of two exit reasons:
   case KVM_EXIT_EXCEPTION:
       What exception will cause KVM exit?
   default:
            dprintf("kvm_arch_handle_exit\n");
            ret = kvm_arch_handle_exit(env, run);
       What exit reasons are default?

3. How could DMA interrupt the cpu when it finishes and the qemu-kvm
is still running in kvm now?

I am still working in the patch, but these confusions really prevent
me moving forward. Thanks first for you guys giving me more hints.


The following is the code so far I write:
The main idea is synchronizing the CPU state and enter into the
emulator mode when switching from kvm to emulator. I only do the
switch when the exit reason is KVM_EXIT_IRQ_WINDOW_OPEN.

However, I got the following errors:
Whenever switch from kvm to qemu, the interrupt request in qemu will
cause qemu enter into smm mode which is definitely a bug.

This is the code that tries to synchronize the CPU state when the IRQ
Window is open. And then tries to switch back to the QEMU emulator
mode.
+{
+  CPUState *env = mon_get_cpu();
+  env->ask_for_emulation = 0;
+  monitor_printf(mon, "Stop emulation\n");
+}
+
+static void do_is_emulation(Monitor *mon, const QDict *qdict, QObject
**ret_data)
+{
+  CPUState *env = mon_get_cpu();
+  if(env->is_in_emulation)
+    monitor_printf(mon, "Emulating now\n");
+  else
+    monitor_printf(mon, "Virtualizing now\n");
+}
+


Thanks for the help, guys. I appreciate for your time to help!

regards,
Wenhao


On Thu, Apr 1, 2010 at 1:42 AM, Avi Kivity <avi@redhat.com> wrote:
> On 03/31/2010 02:31 PM, Juan Quintela wrote:
>>
>> Wenhao Xu<xuwenhao2008@gmail.com>  wrote:
>>
>>>
>>> Hi, Juan,
>>>    I am fresh to both QEMU and KVM. But so far, I notice that QEMU
>>> uses "KVM_SET_USER_MEMORY_REGION" to set memory region that KVM can
>>> use and uses cpu_register_physical_memory_offset to register the same
>>> memory to QEMU emulator, which means QEMU and KVM use the same host
>>> virtual memory. And therefore the memory KVM modified could be
>>> directly reflected to QEMU. I don't quite understand the different
>>> memory layout problem between the two. So I don't know exactly what
>>> you mean to "fix" it?
>>>
>>
>> 1st. qemu-kvm.git and qemu.git memory layouts are different, indeed with
>> qemu.git kvm mode. (yes it is complex and weird).
>>
>> kvm vs qemu initialization is different.  Expecting to stop kvm, and run
>> tcg from there is not going to work.  I guess it would need a lot of
>> changes, but I haven't looked at it myself.
>>
>
> I don't think it's so far fetched.  In fact early versions of qemu-kvm
> switched between emulation and virtualization (emulate until 64-bit mode,
> also emulate mmio instructions in qemu).
>
> Even today, all memory initialization is done via generic qemu mechanisms.
>  So long as you synchronize all state (pit, irqchip, registers) you should
> be fine.
>
>>>    For why switching is useful? Actually, I am a master student now
>>> and doing a course project. What am I arguing is that QEMU could be
>>> potentially useful to do many instrumentation analysis, but it is a
>>> bit slow. So by combing with KVM, when the os runs to some place where
>>> we are interested in, switch it to QEMU emulator mode and do the
>>> analysis and then switch back.
>>>
>>
>> idea is good, but I don't think that it is _so_ easy at this point.  tcg
>> and kvm basically live in a different world.  Not sure of what needs to
>> be done to make them back on sync.
>>
>
> cpu_synchronize_state()
>
> --
> error compiling committee.c: too many arguments to function
>
>

On 04/01/2010 10:35 PM, Wenhao Xu wrote:
> Does current qemu-kvm (qemu v0.12.3) use the irqchip, pit of KVM? I
> cannot find any KVM_CREATE_IRQCHIP and KVM_CREATE_PIT in the qemu
> code.
>
>    

Are you looking at qemu or qemu-kvm?

> Concerning the interface between qemu and kvm, I have the following confusion:
> 1. How irqchip and pit of KVM collaborating with the irq and pit
> emulation of QEMU? As far as I see, qemu-kvm still uses qemu's irq and
> pit emulation, doesn't it?
>    

No, they're completely separate.

> 2. For return from KVM to QEMU, I cannot get the meaning of two exit reasons:
>     case KVM_EXIT_EXCEPTION:
>         What exception will cause KVM exit?
>    

I think that's obsolete.

>     default:
>              dprintf("kvm_arch_handle_exit\n");
>              ret = kvm_arch_handle_exit(env, run);
>         What exit reasons are default?
>
> 3. How could DMA interrupt the cpu when it finishes and the qemu-kvm
> is still running in kvm now?
>    

Usually the device that does the dma will raise an interrupt, which qemu 
is waiting for.

> I am still working in the patch, but these confusions really prevent
> me moving forward. Thanks first for you guys giving me more hints.
>
>
> The following is the code so far I write:
> The main idea is synchronizing the CPU state and enter into the
> emulator mode when switching from kvm to emulator. I only do the
> switch when the exit reason is KVM_EXIT_IRQ_WINDOW_OPEN.
>    

That doesn't happen with qemu-kvm.

> However, I got the following errors:
> Whenever switch from kvm to qemu, the interrupt request in qemu will
> cause qemu enter into smm mode which is definitely a bug.
>    

Definitely shouldn't happen.