linux-kernel - Re: [PATCH 2/5] KVM: x86: drop fpu

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Date:	Mon, 18 Aug 2014 13:26:41 +0300
From:	Avi Kivity <avi.kivity@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Gleb Natapov <gleb@...nel.org>, hpa@...or.com, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] KVM: x86: drop fpu_activate hook


On 08/18/2014 01:20 PM, Paolo Bonzini wrote:
> Il 18/08/2014 11:50, Wanpeng Li ha scritto:
>> fpu_activate hook is introduced by commit 6b52d186 (KVM: Activate fpu on
>> clts), however, there is no user currently, this patch drop it.
>>
>> Reviewed-by: Yang Zhang <yang.z.zhang@...el.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
>> ---
>>   arch/x86/include/asm/kvm_host.h | 1 -
>>   arch/x86/kvm/svm.c              | 1 -
>>   arch/x86/kvm/vmx.c              | 1 -
>>   3 files changed, 3 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 5724601..b68f3e5 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -710,7 +710,6 @@ struct kvm_x86_ops {
>>   	void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
>>   	unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
>>   	void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
>> -	void (*fpu_activate)(struct kvm_vcpu *vcpu);
>>   	void (*fpu_deactivate)(struct kvm_vcpu *vcpu);
>>   
>>   	void (*tlb_flush)(struct kvm_vcpu *vcpu);
>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
>> index ddf7427..1f49c86 100644
>> --- a/arch/x86/kvm/svm.c
>> +++ b/arch/x86/kvm/svm.c
>> @@ -4349,7 +4349,6 @@ static struct kvm_x86_ops svm_x86_ops = {
>>   	.cache_reg = svm_cache_reg,
>>   	.get_rflags = svm_get_rflags,
>>   	.set_rflags = svm_set_rflags,
>> -	.fpu_activate = svm_fpu_activate,
>>   	.fpu_deactivate = svm_fpu_deactivate,
>>   
>>   	.tlb_flush = svm_flush_tlb,
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 71cbee5..2963303 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8896,7 +8896,6 @@ static struct kvm_x86_ops vmx_x86_ops = {
>>   	.cache_reg = vmx_cache_reg,
>>   	.get_rflags = vmx_get_rflags,
>>   	.set_rflags = vmx_set_rflags,
>> -	.fpu_activate = vmx_fpu_activate,
>>   	.fpu_deactivate = vmx_fpu_deactivate,
>>   
>>   	.tlb_flush = vmx_flush_tlb,
>>
> Avi/Gleb, do you remember any particular reason for this?
>

IIRC (vaguely) if we expect the fpu to be used in the near future, we 
activate it eagerly so that we don't fault when it is used.

Prevents the sequence:

guest user: use fpu
#NM
host: reflect #NM to guest
guest kernel: CLTS
guest kernel: switch fpu state
#NM
host: switch fpu
guest kernel: switch fpu state (restarted)
guest user: use fpu (restarted)

Why was the user removed? Full-time eager fpu?
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