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

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Date:	Mon, 18 Aug 2014 12:51:00 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Avi Kivity <avi.kivity@...il.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

Il 18/08/2014 12:26, Avi Kivity ha scritto:
> 
> 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?

No, I mean any reason to keep the hooks.  In the meanwhile I found it myself:

commit 2d04a05bd7e93c13f13a82ac40de4065a99d069b
Author: Avi Kivity <avi@...hat.com>
Date:   Wed Apr 20 15:32:49 2011 +0300

    KVM: x86 emulator: emulate CLTS internally
    
    Avoid using ctxt->vcpu; we can do everything with ->get_cr() and ->set_cr().
    
    A side effect is that we no longer activate the fpu on emulated CLTS; but that
    should be very rare.
    
    Signed-off-by: Avi Kivity <avi@...hat.com>

vmx_fpu_activate and svm_fpu_activate are still called on #NM and CLTS, but
never from common code after the above patch.

Activation on CLTS is currently VMX only; I guess on AMD we could check the
decode assists' CR_VALID bit and instruction length to detect CLTS.

Paolo
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