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

Paolo Bonzini
2015-04-10T08:41:17Z

On 09/04/2015 18:53, 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).

Good idea! Does ARM have a way to do "lazy" save/restore? On x86 we
know if the FP/SIMD state was in use on the host, and we can force an
exit the first time the guest uses the FP/SIMD.

On ARM, it wouldn't be a problem to handle this kind of exit straight in
EL2 (similar to the hack Ard used when playing with memory attributes).

Paolo

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.
>
> 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);
> 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
> + * Author: Marc Zyngier <marc.zyngier@arm.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/linkage.h>
> +
> +#include <asm/asm-offsets.h>
> +#include <asm/fpsimdmacros.h>
> +
> +#define CPU_GP_REG_OFFSET(x) (CPU_GP_REGS + x)
> +
> +ENTRY(__kvm_save_fpsimd)
> + // x0: cpu context address
> + // x1, x2: tmp regs
> + add x1, x0, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> + fpsimd_save x1, 2
> + ret
> +END(__kvm_save_fpsimd)
> +
> +ENTRY(__kvm_restore_fpsimd)
> + // x0: cpu context address
> + // x1, x2: tmp regs
> + add x1, x0, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> + fpsimd_restore x1, 2
> + ret
> +END(__kvm_restore_fpsimd)
> diff --git a/arch/arm64/kvm/handle_fpsimd.c b/arch/arm64/kvm/handle_fpsimd.c
> new file mode 100644
> index 0000000..3d34cc9
> --- /dev/null
> +++ b/arch/arm64/kvm/handle_fpsimd.c
> @@ -0,0 +1,42 @@
> +/*
> + * Copyright (C) 2015 - ARM Ltd
> + * Author: Marc Zyngier <marc.zyngier@arm.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/kvm_host.h>
> +
> +void kvm_fpsimd_flush_hwstate(struct kvm_vcpu *vcpu)
> +{
> + unsigned long flags;
> +
> + local_irq_save(flags);
> +
> + __kvm_save_fpsimd(vcpu->arch.host_cpu_context);
> + __kvm_restore_fpsimd(&vcpu->arch.ctxt);
> +
> + local_irq_restore(flags);
> +}
> +
> +void kvm_fpsimd_sync_hwstate(struct kvm_vcpu *vcpu)
> +{
> + unsigned long flags;
> +
> + local_irq_save(flags);
> +
> + __kvm_save_fpsimd(&vcpu->arch.ctxt);
> + __kvm_restore_fpsimd(vcpu->arch.host_cpu_context);
> +
> + local_irq_restore(flags);
> +}
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index 5befd01..425c1ad 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -21,7 +21,6 @@
> #include <asm/assembler.h>
> #include <asm/debug-monitors.h>
> #include <asm/esr.h>
> -#include <asm/fpsimdmacros.h>
> #include <asm/kvm.h>
> #include <asm/kvm_arm.h>
> #include <asm/kvm_asm.h>
> @@ -102,20 +101,6 @@
> restore_common_regs
> .endm
>
> -.macro save_fpsimd
> - // x2: cpu context address
> - // x3, x4: tmp regs
> - add x3, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> - fpsimd_save x3, 4
> -.endm
> -
> -.macro restore_fpsimd
> - // x2: cpu context address
> - // x3, x4: tmp regs
> - add x3, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> - fpsimd_restore x3, 4
> -.endm
> -
> .macro save_guest_regs
> // x0 is the vcpu address
> // x1 is the return code, do not corrupt!
> @@ -904,14 +889,6 @@ __restore_debug:
> restore_debug
> ret
>
> -__save_fpsimd:
> - save_fpsimd
> - ret
> -
> -__restore_fpsimd:
> - restore_fpsimd
> - ret
> -
> /*
> * u64 __kvm_vcpu_run(struct kvm_vcpu *vcpu);
> *
> @@ -932,7 +909,6 @@ ENTRY(__kvm_vcpu_run)
> kern_hyp_va x2
>
> save_host_regs
> - bl __save_fpsimd
> bl __save_sysregs
>
> compute_debug_state 1f
> @@ -948,7 +924,6 @@ ENTRY(__kvm_vcpu_run)
> add x2, x0, #VCPU_CONTEXT
>
> bl __restore_sysregs
> - bl __restore_fpsimd
>
> skip_debug_state x3, 1f
> bl __restore_debug
> @@ -967,7 +942,6 @@ __kvm_vcpu_return:
> add x2, x0, #VCPU_CONTEXT
>
> save_guest_regs
> - bl __save_fpsimd
> bl __save_sysregs
>
> skip_debug_state x3, 1f
> @@ -986,7 +960,6 @@ __kvm_vcpu_return:
> kern_hyp_va x2
>
> bl __restore_sysregs
> - bl __restore_fpsimd
>
> skip_debug_state x3, 1f
> // Clear the dirty flag for the next run, as all the state has
>