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

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Date:	Wed, 20 May 2015 07:14:50 +0000
From:	"Li, Liang Z" <liang.z.li@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"Zhang, Yang Z" <yang.z.zhang@...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

> > 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 for your comments.  I will change the code according to your suggestion.


> Thanks,
> 
> Paolo

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