From: Yang Zhang <yang.zhang.wz@gmail.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
Xiao Guangrong <guangrong.xiao@linux.intel.com>
Subject: Re: [PATCH 1/2] KVM: x86: disable MPX if host did not enable MPX XSAVE features
Date: Thu, 24 Mar 2016 21:06:27 +0800 [thread overview]
Message-ID: <56F3E653.6040901@gmail.com> (raw)
In-Reply-To: <56E2B58B.8020605@redhat.com>
On 2016/3/11 20:09, Paolo Bonzini wrote:
Sorry for the late reply.
>
>
> On 11/03/2016 03:37, Yang Zhang wrote:
>>> @@ -97,7 +104,7 @@ 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);
>>>
>>> - vcpu->arch.eager_fpu = use_eager_fpu() || guest_cpuid_has_mpx(vcpu);
>>> + vcpu->arch.eager_fpu = use_eager_fpu();
>>
>> Hi Paolo,
>>
>> As i mentioned on another thread, force KVM to use eager fpu
>> unconditionally may introduce the performance regression. Though the
>> cost for eager fpu is very small especially in modern CPU, it still
>> cannot be ignored on old platform.
>
> This patch doesn't change anything in that respect. It doesn't enable
> eager FPU in any case where it wasn't already enabled before the patch.
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);
Besides, vmx_fpu_activate is called when do vcpu_reset. So it seems no
need to call fpu_activate() here again.
--
best regards
yang
next prev parent reply other threads:[~2016-03-24 13:06 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-08 11:44 [PATCH 0/2] KVM: x86: disable MPX if host did not enable MPX XSAVE features Paolo Bonzini
2016-03-08 11:44 ` [PATCH 1/2] " Paolo Bonzini
[not found] ` <CA+3C=r-NVn__gPVdYRSypzjfejRBgB4VVY5OpckS2QGjXnUu2Q@mail.gmail.com>
2016-03-11 12:09 ` Paolo Bonzini
2016-03-24 13:06 ` Yang Zhang [this message]
2016-03-24 13:07 ` Paolo Bonzini
2016-04-13 9:47 ` Yang Zhang
2016-05-10 12:38 ` Paolo Bonzini
2016-03-08 11:44 ` [PATCH 2/2] KVM: x86: remove eager_fpu field of struct kvm_vcpu_arch Paolo Bonzini
2016-03-10 12:28 ` [PATCH 0/2] KVM: x86: disable MPX if host did not enable MPX XSAVE features Xiao Guangrong
2016-03-10 12:30 ` Paolo Bonzini
2016-03-10 17:35 ` Ingo Molnar
2016-03-10 17:38 ` Paolo Bonzini
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