Re: [PATCH] kvm/fpu: Enable fully eager restore kvm FPU

[Avi Kivity <avi@redhat.com>]

On 08/16/2012 08:14 AM, Xudong Hao wrote:
> Enable KVM FPU fully eager restore, if there is other FPU state which isn't tracked by
> CR0.TS bit.
> 
> Tested with these cases:
> 1) SpecCPU2000 workload( 1 VM, 2 VMs)
> 2) Program for floating point caculate

Is the motivation performance or correctness?

> +
>  struct kvm_memory_alias {
>  	__u32 slot;  /* this has a different namespace than memory slots */
>  	__u32 flags;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b6379e5..2e628e5 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5966,7 +5966,18 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>  	vcpu->guest_fpu_loaded = 0;
>  	fpu_save_init(&vcpu->arch.guest_fpu);
>  	++vcpu->stat.fpu_reload;
> -	kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
> +	/*
> +	 * Currently KVM trigger FPU restore by #NM (via CR0.TS),
> +	 * till now only XCR0.bit0, XCR0.bit1, XCR0.bit2 is tracked
> +	 * by TS bit, there might be other FPU state is not tracked
> +	 * by TS bit. 

Which state is that?

> Here it only make FPU deactivate request and do 
> +	 * FPU lazy restore for these cases: 1)xsave isn't enabled 
> +	 * in guest, 2)all guest FPU states can be tracked by TS bit.
> +	 * For others, doing fully FPU eager restore.
> +	 */
> +	if (!kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) ||
> +		!(vcpu->arch.xcr0 & ~KVM_XSTATE_LAZY))
> +		kvm_make_request(KVM_REQ_DEACTIVATE_FPU, vcpu);
>  	trace_kvm_fpu(0);
>  }

Is there no way to track accesses to this extended state?

Although I expect that on modern hardware which exits rarely, eager fpu
reload might be more performant.


-- 
error compiling committee.c: too many arguments to function