linux-kernel - Re: [PATCH] kvm/fpu: Enable eager restore kvm FPU for MPX

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Date:	Wed, 20 May 2015 08:41:42 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	"Li, Liang Z" <liang.z.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	"gleb@...nel.or" <gleb@...nel.or>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH] kvm/fpu: Enable eager restore kvm FPU for MPX



On 20/05/2015 07:20, Zhang, Yang Z wrote:
> Li, Liang Z wrote on 2015-05-20:
>> The MPX feature requires eager KVM FPU restore support. We have
>> verified that MPX cannot work correctly with the current lazy KVM FPU
>> restore mechanism. Eager KVM FPU restore should be enabled if the MPX
>> feature is exposed to VM.
>>
>> Signed-off-by: Liang Li <liang.z.li@...el.com>
>> ---
>>  arch/x86/kvm/vmx.c | 2 ++
>>  arch/x86/kvm/x86.c | 3 ++-
>>  2 files changed, 4 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index f7b6168..e2cccbe 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8445,6 +8445,8 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>>  			goto free_vmcs;
>>  	}
>>  
>> +	if (vmx_mpx_supported())
>> +		vmx_fpu_activate(&vmx->vcpu);
>>  	return &vmx->vcpu;
>>  
>>  free_vmcs:
> 
> Is it better to use guest_cpuid_has_mpx() instead of vmx_mpx_supported()?

CPUID hasn't been set yet, so I think it is okay to key it on
vmx_mpx_supported().  It will be deactivated soon afterwards.

Or even do it unconditionally; just make sure to add a comment about it.

>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 5f38188..5993f5f
>> 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7060,7 +7060,8 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>>  	fpu_save_init(&vcpu->arch.guest_fpu);
>>  	__kernel_fpu_end();
>>  	++vcpu->stat.fpu_reload;
>> -	kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
>> +	if (!kvm_x86_ops->mpx_supported())
>> +		kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
>>  	trace_kvm_fpu(0);
>>  }

This is a hotter path.  Here it's definitely better to avoid the call to
kvm_x86_ops->mpx_supported().  Especially because, with MPX enabled, you
would call this on every userspace exit.

Yang's suggestion of using CPUID is actually more valuable here.  You
could add a new field eager_fpu in kvm->arch and update it in
kvm_update_cpuid.

Thanks,

Paolo
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