linux-kernel - Re: [PATCH] KVM: X86: Don't load/put guest FPU context for sleeping AP

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Date:   Tue, 10 Mar 2020 09:01:29 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: X86: Don't load/put guest FPU context for sleeping
 AP

On Tue, Mar 10, 2020 at 03:01:19PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> kvm_load_guest_fpu() and kvm_put_guest_fpu() each consume more than 14us 
> observed by ftrace, the qemu userspace FPU is swapped out for the guest 
> FPU context for the duration of the KVM_RUN ioctl even if sleeping AP, 
> we shouldn't load/put guest FPU context for this case especially for 
> serverless scenario which sensitives to boot time.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/x86.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5de2006..080ffa4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8680,7 +8680,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  
>  	vcpu_load(vcpu);
>  	kvm_sigset_activate(vcpu);
> -	kvm_load_guest_fpu(vcpu);
>  
>  	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
>  		if (kvm_run->immediate_exit) {
> @@ -8718,12 +8717,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  		}
>  	}
>  
> +	kvm_load_guest_fpu(vcpu);

Ugh, so this isn't safe on MPX capable CPUs, kvm_apic_accept_events() can
trigger kvm_vcpu_reset() with @init_event=true and try to unload guest_fpu.

We could hack around that issue, but it'd be ugly, and I'm also concerned
that calling vmx_vcpu_reset() without guest_fpu loaded will be problematic
in the future with all the things that are getting managed by XSAVE.

> +
>  	if (unlikely(vcpu->arch.complete_userspace_io)) {
>  		int (*cui)(struct kvm_vcpu *) = vcpu->arch.complete_userspace_io;
>  		vcpu->arch.complete_userspace_io = NULL;
>  		r = cui(vcpu);
>  		if (r <= 0)
> -			goto out;
> +			goto out_fpu;
>  	} else
>  		WARN_ON(vcpu->arch.pio.count || vcpu->mmio_needed);
>  
> @@ -8732,8 +8733,9 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>  	else
>  		r = vcpu_run(vcpu);
>  
> -out:
> +out_fpu:
>  	kvm_put_guest_fpu(vcpu);
> +out:
>  	if (vcpu->run->kvm_valid_regs)
>  		store_regs(vcpu);
>  	post_kvm_run_save(vcpu);
> -- 
> 2.7.4
>