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SubjectRE: [PATCH] kvm/fpu: Enable eager restore kvm FPU for MPX
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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@intel.com>
>>> ---
>>> 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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