Re: [Qemu-devel] [QEMU PATCH v4 0/10]: target/i386: kvm: Add support for save and restore of nested state

[Liran Alon]

> On 20 Jun 2019, at 15:38, Paolo Bonzini <address@hidden> wrote:
> 
> On 19/06/19 18:21, Liran Alon wrote:
>> Hi,
>> 
>> This series aims to add support for QEMU to be able to migrate VMs that
>> are running nested hypervisors. In order to do so, it utilizes the new
>> IOCTLs introduced in KVM commit 8fcc4b5923af ("kvm: nVMX: Introduce
>> KVM_CAP_NESTED_STATE") which was created for this purpose.
> 
> Applied with just three minor changes that should be uncontroversial:

ACK. Where can I see the applied patches for review?

> 
>> 6rd patch updates linux-headers to have updated struct kvm_nested_state.
>> The updated struct now have explicit fields for the data portion.
> 
> Changed patch title to "linux-headers: sync with latest KVM headers from
> Linux 5.2”

ACK.

> 
>> 7rd patch add vmstate support for saving/restoring kernel integer types 
>> (e.g. __u16).
>> 
>> 8th patch adds support for saving and restoring nested state in order to 
>> migrate
>> guests which run a nested hypervisor.
> 
> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> index e924663f32..f3cf6e1b27 100644
> --- a/target/i386/kvm.c
> +++ b/target/i386/kvm.c
> @@ -1671,10 +1671,10 @@ int kvm_arch_init_vcpu(CPUState *cs)
>             struct kvm_vmx_nested_state_hdr *vmx_hdr =
>                 &env->nested_state->hdr.vmx;
> 
> +            env->nested_state->format = KVM_STATE_NESTED_FORMAT_VMX;
>             vmx_hdr->vmxon_pa = -1ull;
>             vmx_hdr->vmcs12_pa = -1ull;
>         }
> -
>     }
> 
>     cpu->kvm_msr_buf = g_malloc0(MSR_BUF_SIZE);
> 
> which is a no-op since KVM_STATE_NESTED_FORMAT_VMX is zero, but it's tidy.

I agree. My bad. Thanks for adding this :)

> 
>> 9th patch add support for KVM_CAP_EXCEPTION_PAYLOAD. This new KVM capability
>> allows userspace to properly distingiush between pending and injecting 
>> exceptions.
>> 
>> 10th patch changes the nested virtualization migration blocker to only
>> be added when kernel lack support for one of the capabilities required
>> for correct nested migration. i.e. Either KVM_CAP_NESTED_STATE or
>> KVM_CAP_EXCEPTION_PAYLOAD.
> 
> Had to disable this for SVM unfortunately.

For backwards compatibility I assume… Sounds reasonable to me so ACK.

Even though I must say I would really like to hear your opinion about the 
thread I had with David Gilbert regarding QEMU’s migration backwards 
compatibility:
https://www.mail-archive.com/address@hidden/msg622274.html

Thanks for the assistance pushing this forward,
-Liran