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


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.

>> 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.

Thanks,

Paolo


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