Re: [RFC PATCH] arm64: KVM: remove fpsimd save/restore from the world switch

Christoffer Dall
2015-04-13T12:57:51Z

On Thu, Apr 09, 2015 at 05:53:59PM +0100, Marc Zyngier wrote:
> The world switch spends quite some time dealing with the FP/SIMD
> registers, as the state is quite sizeable (32 128bit registers,
> plus some crumbs on the side). We save/restore them on each
> entry/exit, so that both the host and the guest always see
> the state they expect.
>
> But let's face it: the host kernel doesn't care. It is the host
> userspace that actually cares about FP. An obvious improvement is
> to remove the save/restore from the world switch, and only perform
> it when we're about to enter/exit the guest (by plugging it into
> vcpu_load/vcpu_put). The effect is pretty spectacular when running
> hackbench (which is the only benchmark worth looking at):
>
> Without this patch:
>
> Running with 50*40 (== 2000) tasks.
> Time: 36.756
> Running with 50*40 (== 2000) tasks.
> Time: 36.679
> Running with 50*40 (== 2000) tasks.
> Time: 36.699
>
> With this patch:
>
> Running with 50*40 (== 2000) tasks.
> Time: 30.947
> Running with 50*40 (== 2000) tasks.
> Time: 30.868
> Running with 50*40 (== 2000) tasks.
> Time: 30.961
>
> This is on a HiKey board (8*A53), with a 4 vcpu guest.

cool. Based on stats from kvm-unit-tests on A57 we should reduce the
overall world-switch cost (in the best cases, caches, etc.) with ~8.5%,
but this is even better and we are doing slightly more work than
context-switching here, so I'm guessing factoring in potential extra
cache misses, it can be this good.

However, on XGene with Ubuntu 14.04 Trusty, I get the following (do not
compare to Marc's results, I may be using different kernel settings and
different payload size):

Without the patch:

Running with 50*40 (== 2000) tasks.
Time: 15.970
Running with 50*40 (== 2000) tasks.
Time: 15.963
Running with 50*40 (== 2000) tasks.
Time: 15.875

With the patch:

Running with 50*40 (== 2000) tasks.
Time: 16.768:
Running with 50*40 (== 2000) tasks.
Time: 14.865
Running with 50*40 (== 2000) tasks.
Time: 14.880

On an HP Moonshot server I ran a number of other benchmarks and got
similarly boring results.

Comments on the patch itself below:

>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm/include/asm/kvm_host.h | 3 +++
> arch/arm/kvm/arm.c | 2 ++
> arch/arm64/include/asm/kvm_asm.h | 4 ++++
> arch/arm64/include/asm/kvm_host.h | 3 +++
> arch/arm64/kvm/Makefile | 1 +
> arch/arm64/kvm/fpsimd.S | 39 ++++++++++++++++++++++++++++++++++++
> arch/arm64/kvm/handle_fpsimd.c | 42 +++++++++++++++++++++++++++++++++++++++
> arch/arm64/kvm/hyp.S | 27 -------------------------
> 8 files changed, 94 insertions(+), 27 deletions(-)
> create mode 100644 arch/arm64/kvm/fpsimd.S
> create mode 100644 arch/arm64/kvm/handle_fpsimd.c
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index d71607c..65cf1d1 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -226,6 +226,9 @@ static inline void vgic_arch_setup(const struct vgic_params *vgic)
> int kvm_perf_init(void);
> int kvm_perf_teardown(void);
>
> +static inline void kvm_fpsimd_flush_hwstate(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_fpsimd_sync_hwstate(struct kvm_vcpu *vcpu) {}
> +
> void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot);
>
> struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 6f53645..ff1213c 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -287,6 +287,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> vcpu->cpu = cpu;
> vcpu->arch.host_cpu_context = this_cpu_ptr(kvm_host_cpu_state);
>
> + kvm_fpsimd_flush_hwstate(vcpu);

not sure about the flus/sync terminology here, because we're not really
flushing a software model to hardware state - we're doing both in every
step.

How about:

kvm_fpsimd_load_vcpu_state()
kvm_fpsimd_put_vcpu_state()

> kvm_arm_set_running_vcpu(vcpu);
> }
>
> @@ -299,6 +300,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> */
> vcpu->cpu = -1;
>
> + kvm_fpsimd_sync_hwstate(vcpu);
> kvm_arm_set_running_vcpu(NULL);
> }
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index 4f7310f..eafb0c3 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -137,6 +137,10 @@ extern char __restore_vgic_v2_state[];
> extern char __save_vgic_v3_state[];
> extern char __restore_vgic_v3_state[];
>
> +struct kvm_cpu_context;
> +extern void __kvm_save_fpsimd(struct kvm_cpu_context *);
> +extern void __kvm_restore_fpsimd(struct kvm_cpu_context *);
> +
> #endif
>
> #endif /* __ARM_KVM_ASM_H__ */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index f0f58c9..2b968e5 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -201,6 +201,9 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
> int kvm_perf_init(void);
> int kvm_perf_teardown(void);
>
> +void kvm_fpsimd_flush_hwstate(struct kvm_vcpu *vcpu);
> +void kvm_fpsimd_sync_hwstate(struct kvm_vcpu *vcpu);
> +
> struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
>
> static inline void __cpu_init_hyp_mode(phys_addr_t boot_pgd_ptr,
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index d5904f8..6d9c2b7 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -18,6 +18,7 @@ kvm-$(CONFIG_KVM_ARM_HOST) += $(ARM)/psci.o $(ARM)/perf.o
> kvm-$(CONFIG_KVM_ARM_HOST) += emulate.o inject_fault.o regmap.o
> kvm-$(CONFIG_KVM_ARM_HOST) += hyp.o hyp-init.o handle_exit.o
> kvm-$(CONFIG_KVM_ARM_HOST) += guest.o reset.o sys_regs.o sys_regs_generic_v8.o
> +kvm-$(CONFIG_KVM_ARM_HOST) += fpsimd.o handle_fpsimd.o
>
> kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/vgic.o
> kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/vgic-v2.o
> diff --git a/arch/arm64/kvm/fpsimd.S b/arch/arm64/kvm/fpsimd.S
> new file mode 100644
> index 0000000..458a1a7
> --- /dev/null
> +++ b/arch/arm64/kvm/fpsimd.S
> @@ -0,0 +1,39 @@
> +/*
> + * Copyright (C) 2012,2013 - ARM Ltd

I don't know if you care, but shouldn't this be 2015?

Otherwise looks good!
-Christoffer