Subject: [RFC PATCH v2 3/3] KVM: arm64: Optimise FPSIMD handling to reduce guest/host thrashing

From: [email-masked] (Christoffer Dall)
Date: Mon, 9 Apr 2018 11:48:18 +0200

On Fri, Apr 06, 2018 at 04:01:04PM +0100, Dave Martin wrote:
> This patch refactors KVM to align the host and guest FPSIMD
> save/restore logic with each other for arm64.  This reduces the
> number of redundant save/restore operations that must occur, and
> reduces the common-case IRQ blackout time during guest exit storms
> by saving the host state lazily and optimising away the need to
> restore the host state before returning to the run loop.
> 
> Four hooks are defined in order to enable this:
> 
>  * kvm_arch_vcpu_run_map_fp():
>    Called on PID change to map necessary bits of current to Hyp.
> 
>  * kvm_arch_vcpu_load_fp():
>    Set up FP/SIMD for entering the KVM run loop (parse as
>    "vcpu_load fp").
> 
>  * kvm_arch_vcpu_park_fp():
>    Get FP/SIMD into a safe state for re-enabling interrupts after a
>    guest exit back to the run loop.
> 
>  * kvm_arch_vcpu_put_fp():
>    Save guest FP/SIMD state back to memory and dissociate from the
>    CPU ("vcpu_put fp").
> 
> Also, the arm64 FPSIMD context switch code is updated to enable it
> to save back FPSIMD state for a vcpu, not just current.  A few
> helpers drive this:
> 
>  * fpsimd_bind_state_to_cpu(struct user_fpsimd_state *fp):
>    mark this CPU as having context fp (which may belong to a vcpu)
>    currently loaded in its registers.  This is the non-task
>    equivalent of the static function fpsimd_bind_to_cpu() in
>    fpsimd.c.
> 
>  * task_fpsimd_save():
>    exported to allow KVM to save the guest's FPSIMD state back to
>    memory on exit from the run loop.
> 
>  * fpsimd_flush_state():
>    invalidate any context's FPSIMD state that is currently loaded.
>    Used to disassociate the vcpu from the CPU regs on run loop exit.
> 
> These changes allow the run loop to enable interrupts (and thus
> softirqs that may use kernel-mode NEON) without having to save the
> guest's FPSIMD state eagerly.
> 
> Some new vcpu_arch fields are added to make all this work.  Because
> host FPSIMD state can now be saved back directly into current's
> thread_struct as appropriate, host_cpu_context is no longer used
> for preserving the FPSIMD state.  However, it is still needed for
> preserving other things such as the host's system registers.  To
> avoid ABI churn, the redundant storage space in host_cpu_context is
> not removed for now.
> 
> arch/arm is not addressed by this patch and continues to use its
> current save/restore logic.  It could provide implementations of
> the helpers later if desired.
> 
> Signed-off-by: Dave Martin <[email-masked]>
> ---
>  arch/arm/include/asm/kvm_host.h   |  8 +++++++
>  arch/arm64/include/asm/fpsimd.h   |  5 +++++
>  arch/arm64/include/asm/kvm_host.h | 18 +++++++++++++++
>  arch/arm64/kernel/fpsimd.c        | 31 ++++++++++++++++++++------
>  arch/arm64/kvm/Makefile           |  2 +-
>  arch/arm64/kvm/hyp/switch.c       | 46 ++++++++++++++++++++++++---------------
>  virt/kvm/arm/arm.c                | 14 ++++--------
>  7 files changed, 89 insertions(+), 35 deletions(-)
> 
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 248b930..11cd64a3 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -303,6 +303,14 @@ int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
>  int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>  			       struct kvm_device_attr *attr);
>  
> +/*
> + * VFP/NEON switching is all done by the hyp switch code, so no need to
> + * coordinate with host context handling for this state:
> + */
> +static inline void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_arch_vcpu_park_fp(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu) {}
> +
>  /* All host FP/SIMD state is restored on guest exit, so nothing to save: */
>  static inline void kvm_fpsimd_flush_cpu_state(void) {}
>  
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index 1bfc920..dbe7340 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -40,6 +40,8 @@ struct task_struct;
>  extern void fpsimd_save_state(struct user_fpsimd_state *state);
>  extern void fpsimd_load_state(struct user_fpsimd_state *state);
>  
> +extern void task_fpsimd_save(void);
> +
>  extern void fpsimd_thread_switch(struct task_struct *next);
>  extern void fpsimd_flush_thread(void);
>  
> @@ -48,7 +50,10 @@ extern void fpsimd_preserve_current_state(void);
>  extern void fpsimd_restore_current_state(void);
>  extern void fpsimd_update_current_state(struct user_fpsimd_state const *state);
>  
> +extern void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *state);
> +
>  extern void fpsimd_flush_task_state(struct task_struct *target);
> +extern void fpsimd_flush_cpu_state(void);
>  extern void sve_flush_cpu_state(void);
>  
>  /* Maximum VL that SVE VL-agnostic software can transparently support */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 596f8e4..80716fe 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -30,6 +30,7 @@
>  #include <asm/kvm.h>
>  #include <asm/kvm_asm.h>
>  #include <asm/kvm_mmio.h>
> +#include <asm/thread_info.h>
>  
>  #define __KVM_HAVE_ARCH_INTC_INITIALIZED
>  
> @@ -235,6 +236,12 @@ struct kvm_vcpu_arch {
>  
>  	/* Pointer to host CPU context */
>  	kvm_cpu_context_t *host_cpu_context;
> +
> +	struct thread_info *host_thread_info;	/* hyp VA */
> +	struct user_fpsimd_state *host_fpsimd_state;	/* hyp VA */
> +	bool host_sve_in_use;	/* backup for host TIF_SVE while in guest */
> +	bool fp_enabled;
> +
>  	struct {
>  		/* {Break,watch}point registers */
>  		struct kvm_guest_debug_arch regs;
> @@ -398,6 +405,17 @@ static inline void __cpu_init_stage2(void)
>  		  "PARange is %d bits, unsupported configuration!", parange);
>  }
>  
> +/* Guest/host FPSIMD coordination helpers */
> +int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
> +void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu);
> +void kvm_arch_vcpu_park_fp(struct kvm_vcpu *vcpu);
> +void kvm_arch_vcpu_put_fp(struct kvm_vcpu *vcpu);
> +
> +static inline int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
> +{
> +	return kvm_arch_vcpu_run_map_fp(vcpu);
> +}
> +
>  /*
>   * All host FP/SIMD state is restored on guest exit, so nothing needs
>   * doing here except in the SVE case:
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index db08a54..74c5a46 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -268,13 +268,15 @@ static void task_fpsimd_load(void)
>  }
>  
>  /*
> - * Ensure current's FPSIMD/SVE storage in thread_struct is up to date
> + * Ensure current's FPSIMD/SVE storage in memory is up to date
>   * with respect to the CPU registers.
>   *
>   * Softirqs (and preemption) must be disabled.
>   */
> -static void task_fpsimd_save(void)
> +void task_fpsimd_save(void)
>  {
> +	struct user_fpsimd_state *st = __this_cpu_read(fpsimd_last_state.st);
> +
>  	WARN_ON(!in_softirq() && !irqs_disabled());
>  
>  	if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) {
> @@ -290,10 +292,9 @@ static void task_fpsimd_save(void)
>  				return;
>  			}
>  
> -			sve_save_state(sve_pffr(current),
> -				       &current->thread.fpsimd_state.fpsr);
> +			sve_save_state(sve_pffr(current), &st->fpsr);
>  		} else
> -			fpsimd_save_state(&current->thread.fpsimd_state);
> +			fpsimd_save_state(st);
>  	}
>  }
>  
> @@ -1010,6 +1011,17 @@ static void fpsimd_bind_to_cpu(void)
>  	current->thread.fpsimd_cpu = smp_processor_id();
>  }
>  
> +void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *st)
> +{
> +	struct fpsimd_last_state_struct *last =
> +		this_cpu_ptr(&fpsimd_last_state);
> +
> +	WARN_ON(!in_softirq() && !irqs_disabled());
> +
> +	last->st = st;
> +	last->sve_in_use = false;
> +}
> +
>  /*
>   * Load the userland FPSIMD state of 'current' from memory, but only if the
>   * FPSIMD state already held in the registers is /not/ the most recent FPSIMD
> @@ -1054,15 +1066,20 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
>  	local_bh_enable();
>  }
>  
> +void fpsimd_flush_state(unsigned int *cpu)
> +{
> +	*cpu = NR_CPUS;
> +}
> +
>  /*
>   * Invalidate live CPU copies of task t's FPSIMD state
>   */
>  void fpsimd_flush_task_state(struct task_struct *t)
>  {
> -	t->thread.fpsimd_cpu = NR_CPUS;
> +	fpsimd_flush_state(&t->thread.fpsimd_cpu);
>  }
>  
> -static inline void fpsimd_flush_cpu_state(void)
> +void fpsimd_flush_cpu_state(void)
>  {
>  	__this_cpu_write(fpsimd_last_state.st, NULL);
>  }
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index 87c4f7a..36d9c2f 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -19,7 +19,7 @@ kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/psci.o $(KVM)/arm/perf.o
>  kvm-$(CONFIG_KVM_ARM_HOST) += 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 debug.o reset.o sys_regs.o sys_regs_generic_v8.o
> -kvm-$(CONFIG_KVM_ARM_HOST) += vgic-sys-reg-v3.o
> +kvm-$(CONFIG_KVM_ARM_HOST) += vgic-sys-reg-v3.o fpsimd.o
>  kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/aarch32.o
>  
>  kvm-$(CONFIG_KVM_ARM_HOST) += $(KVM)/arm/vgic/vgic.o
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index 8605e04..797b259 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -27,6 +27,7 @@
>  #include <asm/kvm_mmu.h>
>  #include <asm/fpsimd.h>
>  #include <asm/debug-monitors.h>
> +#include <asm/thread_info.h>
>  
>  static bool __hyp_text __fpsimd_enabled_nvhe(void)
>  {
> @@ -47,24 +48,40 @@ bool __hyp_text __fpsimd_enabled(void)
>  	return __fpsimd_is_enabled()();
>  }
>  
> -static void __hyp_text __activate_traps_vhe(void)
> +static bool update_fp_enabled(struct kvm_vcpu *vcpu)
> +{
> +	if (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE) {
> +		vcpu->arch.host_fpsimd_state = NULL;

I can't see where host_fpsimd_state gets set to anything else than NULL,
what am I missing?

Thanks,
-Christoffer