RE: [PATCH] kvm/fpu: Enable eager restore kvm FPU for MPX

[Zhang, Yang Z]

Paolo Bonzini wrote on 2015-05-20:
> 
> 
> On 20/05/2015 07:20, Zhang, Yang Z wrote:
>> Li, Liang Z wrote on 2015-05-20:
>>> The MPX feature requires eager KVM FPU restore support. We have
>>> verified that MPX cannot work correctly with the current lazy KVM
>>> FPU restore mechanism. Eager KVM FPU restore should be enabled if
>>> the MPX feature is exposed to VM.
>>> 
>>> Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
>>> ---
>>>  arch/x86/kvm/vmx.c | 2 ++
>>>  arch/x86/kvm/x86.c | 3 ++-
>>>  2 files changed, 4 insertions(+), 1 deletion(-)
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index
>>> f7b6168..e2cccbe 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -8445,6 +8445,8 @@ static struct kvm_vcpu
>>> *vmx_create_vcpu(struct
> kvm *kvm, unsigned int id)
>>>  			goto free_vmcs;
>>>  	}
>>> +	if (vmx_mpx_supported())
>>> +		vmx_fpu_activate(&vmx->vcpu);
>>>  	return &vmx->vcpu;
>>>  
>>>  free_vmcs:
>> 
>> Is it better to use guest_cpuid_has_mpx() instead of vmx_mpx_supported()?
> 
> CPUID hasn't been set yet, so I think it is okay to key it on
> vmx_mpx_supported().  It will be deactivated soon afterwards.
> 
> Or even do it unconditionally; just make sure to add a comment about it.

Correct! Unconditionally would be acceptable.

> 
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
>>> 5f38188..5993f5f
>>> 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -7060,7 +7060,8 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>>>  	fpu_save_init(&vcpu->arch.guest_fpu);
>>>  	__kernel_fpu_end();
>>>  	++vcpu->stat.fpu_reload;
>>> -	kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
>>> +	if (!kvm_x86_ops->mpx_supported())
>>> +		kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
>>>  	trace_kvm_fpu(0);
>>>  }
> 
> This is a hotter path.  Here it's definitely better to avoid the call
> to kvm_x86_ops->mpx_supported().  Especially because, with MPX
> enabled, you would call this on every userspace exit.
> 
> Yang's suggestion of using CPUID is actually more valuable here.  You
> could add a new field eager_fpu in kvm->arch and update it in kvm_update_cpuid.

Good suggestion!

> 
> Thanks,
> 
> Paolo


Best regards,
Yang


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