linux-kernel - Re: [PATCH 1/2] KVM: x86: disable MPX if host did not enable MPX XSAVE features

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Date:	Tue, 10 May 2016 14:38:38 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Yang Zhang <yang.zhang.wz@...il.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Subject: Re: [PATCH 1/2] KVM: x86: disable MPX if host did not enable MPX
 XSAVE features



On 13/04/2016 11:47, Yang Zhang wrote:
> On 2016/3/24 21:07, Paolo Bonzini wrote:
>>
>>
>> On 24/03/2016 14:06, Yang Zhang wrote:
>>>
>>> I mean why not keep the old way that only activate the eager_fpu while
>>> guest sees the MPX bit in CPUID, like:
>>>
>>> vcpu->arch.eager_fpu = use_eager_fpu() && guest_cpuid_has_mpx(vcpu);
>>
>> If the host uses eager FPU you can assume that it's faster than lazy FPU.
>>
>> Paolo
>>
>>> Besides, vmx_fpu_activate is called when do vcpu_reset. So it seems no
>>> need to call fpu_activate() here again.
> 
> Hi Paolo
> 
> The check in kvm_update_cpuid of fpu is strange since nothing relies on
> cpuid now. How about the following change?

Ok, please post it generated by git-format-patch and with a commit message.

Thanks,

Paolo

> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index bbbaa80..da4e6ad 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -114,9 +114,6 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>         if (best && (best->eax & (F(XSAVES) | F(XSAVEC))))
>                 best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> 
> -       if (use_eager_fpu())
> -               kvm_x86_ops->fpu_activate(vcpu);
> -
>         /*
>          * The existing code assumes virtual address is 48-bit in the
> canonical
>          * address checks; exit if it is ever changed.
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9b7798c..8f57335 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7453,6 +7453,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool
> init_event)
>         vcpu->arch.regs_avail = ~0;
>         vcpu->arch.regs_dirty = ~0;
> 
> +       if (use_eager_fpu())
> +               kvm_x86_ops->fpu_activate(vcpu);
> +
>         kvm_x86_ops->vcpu_reset(vcpu, init_event);
>  }
> 
>