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

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Message-ID: <570E15B9.5070205@gmail.com>
Date:	Wed, 13 Apr 2016 17:47:37 +0800
From:	Yang Zhang <yang.zhang.wz@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.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 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?

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);
  }


-- 
best regards
yang