Re: [Qemu-devel] [PATCH 3/7] KVM: i386: Add support for KVM_CAP_EXCEPTION_PAYLOAD

[Liran Alon]

> On 15 Jun 2019, at 3:57, Liran Alon <address@hidden> wrote:
> 
>> On 15 Jun 2019, at 3:42, Paolo Bonzini <address@hidden> wrote:
>> 
>> From: Liran Alon <address@hidden>
>> 
>> +static bool is_vmx_enabled(CPUX86State *env)
>> +{
>> +    return (IS_INTEL_CPU(env) && (env->cr[4] & CR4_VMXE_MASK));
>> +}
>> +
>> +static bool is_svm_enabled(CPUX86State *env)
>> +{
>> +    return (IS_AMD_CPU(env) && (env->efer & MSR_EFER_SVME));
>> +}
>> +
>> +static bool is_nested_virt_enabled(CPUX86State *env)
>> +{
>> +    return (is_vmx_enabled(env) || is_svm_enabled(env));
>> +}
> 
> I have later realised that this nested_virt_enabled() function is not enough 
> to determine if nested_state is required to be sent.
> This is because it may be that vCPU is running L2 but have momentarily 
> entered SMM due to SMI.
> In this case, CR4 & MSR_EFER are saved in SMRAM and are set to 0 on entering 
> to SMM.
> This means that in case (env->hflags & HF_SMM_MASK), we theoretically should 
> have read saved CR4 & MSR_EFER from SMRAM.
> However, because we cannot reference guest memory at this point (Not valid in 
> case we migrate guest in post-copy), I should change
> code to assume that in case (env->hflags & HF_SMM_MASK), we need to assume 
> that nested-state is needed.
> This should break backwards-compatability migration only in very rare cases 
> and therefore I think it should be sufficient.
> Any objections to this idea?
> 

Actually, this is even worse than I originally thought.
Even in case guest is not currently in SMM mode, if it’s in VMX non-root mode, 
the CR4 read here is L2 CR4. Not L1 CR4.
Therefore, CR4.VMXE doesn’t necessarily indicate if guest have 
nested-virtualization enabled. Same is true for MSR_EFER in case of SVM.

Putting this all together, in case kernel doesn’t support extracting 
nested-state, there is no decent way to know if guest is running 
nested-virtualization.
Which means that in theory we always need to fail migration in case kernel 
doesn’t support KVM_CAP_NESTED_STATE or KVM_CAP_EXCEPTION_PAYLOAD
and vCPU is exposed with VMX/SVM capability.

I can condition this behaviour with a flag that can be manipulated using QMP to 
allow user to indicate it wishes to migrate guest anyway in this case.
This however bring me back to the entire discussion I had with Dr. David Alan 
Gilbert on migration backwards compatibility in general and the fact that I 
believe
we should have a generic QMP command which allows to provide list of VMState 
subsections that can be ignored in migration…
See: https://www.mail-archive.com/address@hidden/msg622274.html

Paolo, What are your thoughts on how I would proceed with this?

-Liran