Re: Where is the entry of hypercalls in kvm?

Anthony Liguori
2010-06-30T15:10:23Z

On 06/30/2010 03:56 AM, Alexander Graf wrote:
> On 30.06.2010, at 10:17, Peter Teoh wrote:
>
>
>> Your questioned is answered here:
>>
>> http://www.spinics.net/lists/kvm/msg37526.html
>>
>> And check this paper out:
>>
>> http://ozlabs.org/~rusty/virtio-spec/virtio-paper.pdf
>>
>> The general concept to remember is that QEMU and KVM just execute the
>> input as binary stream....it does not know what "functions" it is
>> executing...so the binary stream can be any OS (windows / Linux
>> etc)....QEMU just setup the basic block (call basic blocks
>> translation) mechanism, and then execute it block by block. Each
>> block by definition is demarcated by a branch/jump etc. Within the
>> block if there is any privilege instruction, (eg, write MSR registers,
>> load LDT registers etc), then a transition will be made from guest in
>> QEMU into KVM to update the VMCB/VMCS information. (these terms are
>> from Intel/AMD manual).
>>
> Eh, no.
>
> There are two modes of operation:
>
> 1) TCG
> 2) KVM
>
> In mode 1, qemu goes through target-xxx/translate.c and converts the basic blocks you were talking about above to native machine code on the host system using tcg (see the tcg directory). No KVM is involved, everything happens in user mode.
>
> In mode 2, qemu executes _everything_ by calling KVM. There is no guest code interpreted, looked at or whatever in qemu.

Only because there is a mini-x86 interpreter in the kernel. That lets
KVM expose an idealized interface to qemu that requires no instruction
interpretation.

More to the point of the original question, virtio is typically
implemented on top of an emulated PCI device. The kick operation is
implemented as a write to a PCI IO region that's mapped to PIO. If you
look at hw/virtio-pci.c, you'll see the entry points.

Regards,

Anthony Liguori

> Whenever the guest CPU runs, it runs because qemu called ioctrl(VCPU_RUN) on its kvm vcpu fd.
>
>
>> I have not seen any IOCTL calls in QEMU,
>>
> See kvm*.c and target-xxx/kvm.c
>
>
>> but I suspect ultimately it
>> should drop to a VMRUN (for AMD, Intel called it VMLAUNCH or VMRESUME)
>> calls inside KVM, which can be found here:
>>
>> arch/x86/kvm/
>>
>> And the AMD specific virtualization is done in svm.c whereas that of
>> vmx.c is for Intel.
>>
>> Copying the remark in vmx.c:
>>
>> /*
>> * The exit handlers return 1 if the exit was handled fully and guest execution
>> * may resume. Otherwise they set the kvm_run parameter to indicate what needs
>> * to be done to userspace and return 0.
>> */
>> static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu) = {
>> [EXIT_REASON_EXCEPTION_
>>
>> And after reading the Intel manual, u will understand that "exit" here
>> actually refers to the special set of privilege intel instructions,
>> which upon being executed by the guest OS, will immediately caused and
>> VMEXIT condition, and these are handled by the above handler in
>> kvm.ko.
>>
> in kvm-xxx.ko for x86.
>
> Also, please don't top post :)
>
>
> Alex
>
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