[PATCH] arm64: KVM: reduce guest fpsimd trap

[Dave.Martin@arm.com (Dave Martin)]

On Wed, May 16, 2018 at 10:25:40AM +0100, Marc Zyngier wrote:
> [+Dave]
> 
> Hi Nianyao,
> 
> On 16/05/18 10:08, Tangnianyao (ICT) wrote:
> > Add last_fpsimd_trap to notify if guest last exit reason is handling fpsimd. If guest is using fpsimd frequently, save host's fpsimd state and restore guest's fpsimd state and deactive fpsimd trap before returning to guest. It can avoid guest fpsimd trap soon to improve performance.

So, the purpose of this patch is to context switch the FPSIMD state on
initial entry to the guest, instead of enabling the trap and context
switching the FPSIMD state lazily?

And you decide whether to do this or not, based on whether the guest
triggered a lazy FPSIMD switch previously?

> > 
> > Signed-off-by: Nianyao Tang <tangnianyao@huawei.com>
> > ---
> >  arch/arm64/kernel/asm-offsets.c |  1 +
> >  arch/arm64/kvm/hyp/entry.S      |  5 +++++
> >  arch/arm64/kvm/hyp/switch.c     | 38 ++++++++++++++++++++++++++++++++++++--
> >  include/linux/kvm_host.h        |  1 +
> >  4 files changed, 43 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c index 5bdda65..35a9c5c 100644
> > --- a/arch/arm64/kernel/asm-offsets.c
> > +++ b/arch/arm64/kernel/asm-offsets.c
> > @@ -136,6 +136,7 @@ int main(void)
> >  #ifdef CONFIG_KVM_ARM_HOST
> >    DEFINE(VCPU_CONTEXT,		offsetof(struct kvm_vcpu, arch.ctxt));
> >    DEFINE(VCPU_FAULT_DISR,	offsetof(struct kvm_vcpu, arch.fault.disr_el1));
> > +  DEFINE(VCPU_LAST_FPSIMD_TRAP, offsetof(struct kvm_vcpu, 
> > + last_fpsimd_trap));
> >    DEFINE(CPU_GP_REGS,		offsetof(struct kvm_cpu_context, gp_regs));
> >    DEFINE(CPU_USER_PT_REGS,	offsetof(struct kvm_regs, regs));
> >    DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
> > diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S index e41a161..956e042 100644
> > --- a/arch/arm64/kvm/hyp/entry.S
> > +++ b/arch/arm64/kvm/hyp/entry.S
> > @@ -197,6 +197,11 @@ alternative_endif
> >  	add	x0, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> >  	bl	__fpsimd_restore_state
> >  
> > +	// Mark guest using fpsimd now
> > +	ldr x0, [x3, #VCPU_LAST_FPSIMD_TRAP]
> > +	add x0, x0, #1

Can this overflow?

> > +	str x0, [x3, #VCPU_LAST_FPSIMD_TRAP]
> > +
> >  	// Skip restoring fpexc32 for AArch64 guests
> >  	mrs	x1, hcr_el2
> >  	tbnz	x1, #HCR_RW_SHIFT, 1f
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c index d964523..86eea1b 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -92,7 +92,13 @@ static void activate_traps_vhe(struct kvm_vcpu *vcpu)
> >  
> >  	val = read_sysreg(cpacr_el1);
> >  	val |= CPACR_EL1_TTA;
> > -	val &= ~(CPACR_EL1_FPEN | CPACR_EL1_ZEN);
> > +	val &= ~CPACR_EL1_ZEN;
> > +
> > +	if (vcpu->last_fpsimd_trap)
> > +		val |= CPACR_EL1_FPEN;
> > +	else
> > +		val &= ~CPACR_EL1_FPEN;
> > +
> >  	write_sysreg(val, cpacr_el1);
> >  
> >  	write_sysreg(kvm_get_hyp_vector(), vbar_el1); @@ -105,7 +111,13 @@ static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
> >  	__activate_traps_common(vcpu);
> >  
> >  	val = CPTR_EL2_DEFAULT;
> > -	val |= CPTR_EL2_TTA | CPTR_EL2_TFP | CPTR_EL2_TZ;
> > +	val |= CPTR_EL2_TTA | CPTR_EL2_TZ;
> > +
> > +	if (vcpu->last_fpsimd_trap)
> > +		val &= ~CPTR_EL2_TFP;
> > +	else
> > +		val |= CPTR_EL2_TFP;
> > +
> >  	write_sysreg(val, cptr_el2);
> >  }
> >  
> > @@ -406,6 +418,17 @@ int kvm_vcpu_run_vhe(struct kvm_vcpu *vcpu)
> >  	__activate_traps(vcpu);
> >  	__activate_vm(vcpu->kvm);
> >  
> > +	/*
> > +	 * If guest last trap to host for handling fpsimd, last_fpsimd_trap
> > +	 * is set. Restore guest's fpsimd state and deactivate fpsimd trap
> > +	 * to avoid guest traping soon.
> > +	 */
> > +	if (vcpu->last_fpsimd_trap) {
> > +		__fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> > +		__fpsimd_restore_state(&guest_ctxt->gp_regs.fp_regs);
> > +		vcpu->last_fpsimd_trap = 0;
> > +	}
> > +
> >  	sysreg_restore_guest_state_vhe(guest_ctxt);
> >  	__debug_switch_to_guest(vcpu);
> >  
> > @@ -454,6 +477,17 @@ int __hyp_text __kvm_vcpu_run_nvhe(struct kvm_vcpu *vcpu)
> >  	__activate_traps(vcpu);
> >  	__activate_vm(kern_hyp_va(vcpu->kvm));
> >  
> > +	/*
> > +	 * If guest last trap to host for handling fpsimd, last_fpsimd_trap
> > +	 * is set. Restore guest's fpsimd state and deactivate fpsimd trap
> > +	 * to avoid guest traping soon.
> > +	 */
> > +	if (vcpu->last_fpsimd_trap) {
> > +		__fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> > +		__fpsimd_restore_state(&guest_ctxt->gp_regs.fp_regs);
> > +		vcpu->last_fpsimd_trap = 0;
> > +	}
> > +
> >  	__hyp_vgic_restore_state(vcpu);
> >  	__timer_enable_traps(vcpu);
> >  
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 6930c63..46bdf0d 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -274,6 +274,7 @@ struct kvm_vcpu {
> >  	bool preempted;
> >  	struct kvm_vcpu_arch arch;
> >  	struct dentry *debugfs_dentry;
> > +	unsigned int last_fpsimd_trap;
> >  };
> >  
> >  static inline int kvm_vcpu_exiting_guest_mode(struct kvm_vcpu *vcpu)
> > --
> > 2.7.4
> > 
> 
> This doesn't seem to be the 100% correct. I can't see how this works
> when being preempted, for example... I suggest you look at Dave Martin's
> series[1], which does this correctly.

If I've understood correctly, this chooses between eager and lazy
switching, which is addressing a different issue from [1].

In effect, this code is attempting to predict whether the guest will
use FPSIMD before the next exit.  If the prediction is correct and the
guest does use FPSIMD, then the overhead of the lazy FPSIMD trap
disappears.  This is probably a good thing, though it may increase
interrupt latency a little.  However, if the prediction is wrong and
the guest doesn't use FPSIMD before the next exit, then the overhead of
guest entry increases for no benefit, because FPSIMD was switched
unnecessarily.

Do you have any benchmarks or metrics on the accuracy of the
prediction and the overall impact on performance?

The changes in [1] should reduce the number of FPSIMD context switches
overall, so may reduce the benefit of this patch.

Cheers
---Dave