Subject: [PATCH v5 2/3] KVM/arm/arm64: enable enhanced armv7 fp/simd lazy switch

From: [email-masked] (Christoffer Dall)
Date: Fri, 18 Dec 2015 14:49:24 +0100

On Sun, Dec 06, 2015 at 05:07:13PM -0800, Mario Smarduch wrote:
> This patch tracks armv7 fp/simd hardware state with hcptr register.
> On vcpu_load saves host fpexc, enables FP access, and sets trapping
> on fp/simd access. On first fp/simd access trap to handler to save host and 
> restore guest context, clear trapping bits to enable vcpu lazy mode. On 
> vcpu_put if trap bits are cleared save guest and restore host context and 
> always restore host fpexc.
> 
> Signed-off-by: Mario Smarduch <[email-masked]>
> ---
>  arch/arm/include/asm/kvm_emulate.h   | 50 ++++++++++++++++++++++++++++++++++++
>  arch/arm/include/asm/kvm_host.h      |  1 +
>  arch/arm/kvm/Makefile                |  2 +-
>  arch/arm/kvm/arm.c                   | 13 ++++++++++
>  arch/arm/kvm/fpsimd_switch.S         | 46 +++++++++++++++++++++++++++++++++
>  arch/arm/kvm/interrupts.S            | 32 +++++------------------
>  arch/arm/kvm/interrupts_head.S       | 33 ++++++++++--------------
>  arch/arm64/include/asm/kvm_emulate.h |  9 +++++++
>  arch/arm64/include/asm/kvm_host.h    |  1 +
>  9 files changed, 142 insertions(+), 45 deletions(-)
>  create mode 100644 arch/arm/kvm/fpsimd_switch.S
> 
> diff --git a/arch/arm/include/asm/kvm_emulate.h b/arch/arm/include/asm/kvm_emulate.h
> index a9c80a2..3de11a2 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -243,4 +243,54 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
>  	}
>  }
>  
> +#ifdef CONFIG_VFPv3
> +/* Called from vcpu_load - save fpexc and enable guest access to fp/simd unit */

are you really enabling guest access here or just fiddling with fpexc to
ensure you trap accesses to hyp ?

> +static inline void kvm_enable_vcpu_fpexc(struct kvm_vcpu *vcpu)
> +{
> +	u32 fpexc;
> +
> +	asm volatile(
> +	 "mrc p10, 7, %0, cr8, cr0, 0\n"
> +	 "str %0, [%1]\n"
> +	 "mov %0, #(1 << 30)\n"
> +	 "mcr p10, 7, %0, cr8, cr0, 0\n"
> +	 "isb\n"

why do you need an ISB here?  won't there be an implicit one from the
HVC call later before you need this to take effect?

> +	 : "+r" (fpexc)
> +	 : "r" (&vcpu->arch.host_fpexc)
> +	);

this whole bit can be rewritten something like:

fpexc = fmrx(FPEXC);
vcpu->arch.host_fpexc = fpexc;
fpexc |= FPEXC_EN;
fmxr(FPEXC, fpexc);

> +}
> +
> +/* Called from vcpu_put - restore host fpexc */
> +static inline void kvm_restore_host_fpexc(struct kvm_vcpu *vcpu)
> +{
> +	asm volatile(
> +	 "mcr p10, 7, %0, cr8, cr0, 0\n"
> +	 :
> +	 : "r" (vcpu->arch.host_fpexc)
> +	);

similarly here

> +}
> +
> +/* If trap bits are reset then fp/simd registers are dirty */
> +static inline bool kvm_vcpu_vfp_isdirty(struct kvm_vcpu *vcpu)
> +{
> +	return !!(~vcpu->arch.hcptr & (HCPTR_TCP(10) | HCPTR_TCP(11)));

this looks complicated, how about:

return !(vcpu->arch.hcptr & (HCPTR_TCP(10) | HCPTR_TCP(11)));

> +}
> +
> +static inline void vcpu_reset_cptr(struct kvm_vcpu *vcpu)
> +{
> +	vcpu->arch.hcptr |= (HCPTR_TTA | HCPTR_TCP(10)  | HCPTR_TCP(11));
> +}
> +#else
> +static inline void kvm_enable_vcpu_fpexc(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_restore_host_fpexc(struct kvm_vcpu *vcpu) {}
> +static inline bool kvm_vcpu_vfp_isdirty(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +static inline void vcpu_reset_cptr(struct kvm_vcpu *vcpu)
> +{
> +	vcpu->arch.hcptr = HCPTR_TTA;
> +}
> +#endif
> +
>  #endif /* __ARM_KVM_EMULATE_H__ */
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 09bb1f2..ecc883a 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -227,6 +227,7 @@ int kvm_perf_teardown(void);
>  void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot);
>  
>  struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
> +void kvm_restore_host_vfp_state(struct kvm_vcpu *);
>  
>  static inline void kvm_arch_hardware_disable(void) {}
>  static inline void kvm_arch_hardware_unsetup(void) {}
> diff --git a/arch/arm/kvm/Makefile b/arch/arm/kvm/Makefile
> index c5eef02c..411b3e4 100644
> --- a/arch/arm/kvm/Makefile
> +++ b/arch/arm/kvm/Makefile
> @@ -19,7 +19,7 @@ kvm-arm-y = $(KVM)/kvm_main.o $(KVM)/coalesced_mmio.o $(KVM)/eventfd.o $(KVM)/vf
>  
>  obj-y += kvm-arm.o init.o interrupts.o
>  obj-y += arm.o handle_exit.o guest.o mmu.o emulate.o reset.o
> -obj-y += coproc.o coproc_a15.o coproc_a7.o mmio.o psci.o perf.o
> +obj-y += coproc.o coproc_a15.o coproc_a7.o mmio.o psci.o perf.o fpsimd_switch.o
>  obj-y += $(KVM)/arm/vgic.o
>  obj-y += $(KVM)/arm/vgic-v2.o
>  obj-y += $(KVM)/arm/vgic-v2-emul.o
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index dc017ad..1de07ab 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -291,10 +291,23 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  	vcpu->arch.host_cpu_context = this_cpu_ptr(kvm_host_cpu_state);
>  
>  	kvm_arm_set_running_vcpu(vcpu);
> +
> +	/*  Save and enable FPEXC before we load guest context */
> +	kvm_enable_vcpu_fpexc(vcpu);

hmmm, not really sure the 'enable' part of this name is the right choice
when looking at this.  kvm_prepare_vcpu_fpexc ?

> +
> +	/* reset hyp cptr register to trap on tracing and vfp/simd access*/
> +	vcpu_reset_cptr(vcpu);

alternatively you could combine the two functions above into a single
function called something like "vcpu_trap_vfp_enable()" or
"vcpu_load_configure_vfp()"

(I sort of feel like we have reserved the _reset_ namespace for stuff we
actually do at VCPU reset.)

>  }
>  
>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>  {
> +	/* If the fp/simd registers are dirty save guest, restore host. */
> +	if (kvm_vcpu_vfp_isdirty(vcpu))
> +		kvm_restore_host_vfp_state(vcpu);
> +
> +	/* Restore host FPEXC trashed in vcpu_load */
> +	kvm_restore_host_fpexc(vcpu);
> +
>  	/*
>  	 * The arch-generic KVM code expects the cpu field of a vcpu to be -1
>  	 * if the vcpu is no longer assigned to a cpu.  This is used for the
> diff --git a/arch/arm/kvm/fpsimd_switch.S b/arch/arm/kvm/fpsimd_switch.S
> new file mode 100644
> index 0000000..d297c54
> --- /dev/null
> +++ b/arch/arm/kvm/fpsimd_switch.S
> @@ -0,0 +1,46 @@
> +/*
> + * Copyright (C) 2012 - Virtual Open Systems and Columbia University
> + * Author: Christoffer Dall <[email-masked]>

Not quite, this is new code, so you should just claim copyright and
authorship I believe.

> + *
> + * 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.
> + *
> + */
> +#include <linux/linkage.h>
> +#include <linux/const.h>
> +#include <asm/unified.h>
> +#include <asm/page.h>
> +#include <asm/ptrace.h>
> +#include <asm/asm-offsets.h>
> +#include <asm/kvm_asm.h>
> +#include <asm/kvm_arm.h>
> +#include <asm/vfpmacros.h>
> +#include "interrupts_head.S"
> +
> +	.text
> +/**
> +  * void kvm_restore_host_vfp_state(struct vcpu *vcpu) -
> +  *     This function is called from host to save the guest, and restore host
> +  *     fp/simd hardware context. It's placed outside of hyp start/end region.
> +  */
> +ENTRY(kvm_restore_host_vfp_state)
> +#ifdef CONFIG_VFPv3
> +	push	{r4-r7}
> +
> +	add	r7, r0, #VCPU_VFP_GUEST
> +	store_vfp_state r7
> +
> +	add	r7, r0, #VCPU_VFP_HOST
> +	ldr	r7, [r7]
> +	restore_vfp_state r7
> +
> +	pop	{r4-r7}
> +#endif
> +	bx	lr
> +ENDPROC(kvm_restore_host_vfp_state)
> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> index 900ef6d..8e25431 100644
> --- a/arch/arm/kvm/interrupts.S
> +++ b/arch/arm/kvm/interrupts.S
> @@ -116,22 +116,15 @@ ENTRY(__kvm_vcpu_run)
>  	read_cp15_state store_to_vcpu = 0
>  	write_cp15_state read_from_vcpu = 1
>  
> -	@ If the host kernel has not been configured with VFPv3 support,
> -	@ then it is safer if we deny guests from using it as well.
> -#ifdef CONFIG_VFPv3
> -	@ Set FPEXC_EN so the guest doesn't trap floating point instructions
> -	VFPFMRX r2, FPEXC		@ VMRS
> -	push	{r2}
> -	orr	r2, r2, #FPEXC_EN
> -	VFPFMXR FPEXC, r2		@ VMSR
> -#endif
> +	@ Enable tracing and possibly fp/simd trapping

Configure trapping of access to tracing and fp/simd registers

> +	ldr r4, [vcpu, #VCPU_HCPTR]
> +	set_hcptr vmentry, #0, r4

if we store something called HCPTR on the VCPU, then that should really
be HCPTR, so I don't see why we need a macro and this is not just a
write to the HCPTR directly?

>  
>  	@ Configure Hyp-role
>  	configure_hyp_role vmentry
>  
>  	@ Trap coprocessor CRx accesses
>  	set_hstr vmentry
> -	set_hcptr vmentry, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
>  	set_hdcr vmentry
>  
>  	@ Write configured ID register into MIDR alias
> @@ -170,23 +163,12 @@ __kvm_vcpu_return:
>  	@ Don't trap coprocessor accesses for host kernel
>  	set_hstr vmexit
>  	set_hdcr vmexit
> -	set_hcptr vmexit, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11)), after_vfp_restore
>  
> -#ifdef CONFIG_VFPv3
> -	@ Switch VFP/NEON hardware state to the host's
> -	add	r7, vcpu, #VCPU_VFP_GUEST
> -	store_vfp_state r7
> -	add	r7, vcpu, #VCPU_VFP_HOST
> -	ldr	r7, [r7]
> -	restore_vfp_state r7
> +	/* Preserve HCPTR across exits */
> +	mrc     p15, 4, r2, c1, c1, 2
> +	str     r2, [vcpu, #VCPU_HCPTR]

can't you do this in the trap handler so you avoid this on every exit?

>  
> -after_vfp_restore:
> -	@ Restore FPEXC_EN which we clobbered on entry
> -	pop	{r2}
> -	VFPFMXR FPEXC, r2
> -#else
> -after_vfp_restore:
> -#endif
> +	set_hcptr vmexit, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))

again here, I don't think you need a macro, just clear the bits and
store the register.

>  
>  	@ Reset Hyp-role
>  	configure_hyp_role vmexit
> diff --git a/arch/arm/kvm/interrupts_head.S b/arch/arm/kvm/interrupts_head.S
> index 51a5950..7701ccd 100644
> --- a/arch/arm/kvm/interrupts_head.S
> +++ b/arch/arm/kvm/interrupts_head.S
> @@ -593,29 +593,24 @@ ARM_BE8(rev	r6, r6  )
>   * (hardware reset value is 0). Keep previous value in r2.
>   * An ISB is emited on vmexit/vmtrap, but executed on vmexit only if
>   * VFP wasn't already enabled (always executed on vmtrap).
> - * If a label is specified with vmexit, it is branched to if VFP wasn't
> - * enabled.
>   */
> -.macro set_hcptr operation, mask, label = none
> -	mrc	p15, 4, r2, c1, c1, 2
> -	ldr	r3, =\mask
> +.macro set_hcptr operation, mask, reg
> +	mrc     p15, 4, r2, c1, c1, 2
>  	.if \operation == vmentry
> -	orr	r3, r2, r3		@ Trap coproc-accesses defined in mask
> +	mov     r3, \reg              @ Trap coproc-accesses defined in mask
>  	.else
> -	bic	r3, r2, r3		@ Don't trap defined coproc-accesses
> -	.endif
> -	mcr	p15, 4, r3, c1, c1, 2
> -	.if \operation != vmentry
> -	.if \operation == vmexit
> -	tst	r2, #(HCPTR_TCP(10) | HCPTR_TCP(11))
> -	beq	1f
> -	.endif
> -	isb
> -	.if \label != none
> -	b	\label
> -	.endif
> +        ldr     r3, =\mask
> +        bic     r3, r2, r3            @ Don't trap defined coproc-accesses
> +        .endif
> +        mcr     p15, 4, r3, c1, c1, 2
> +        .if \operation != vmentry
> +        .if \operation == vmexit
> +        tst     r2, #(HCPTR_TCP(10) | HCPTR_TCP(11))
> +        beq     1f
> +        .endif
> +        isb
>  1:
> -	.endif
> +        .endif

there are white-space issues here, but I think you can rid of this macro
entirely now.

>  .endm
>  
>  /* Configures the HDCR (Hyp Debug Configuration Register) on entry/return
> diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> index 17e92f0..8dccbd7 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -290,4 +290,13 @@ static inline unsigned long vcpu_data_host_to_guest(struct kvm_vcpu *vcpu,
>  	return data;		/* Leave LE untouched */
>  }
>  
> +static inline void kvm_enable_vcpu_fpexc(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_restore_host_fpexc(struct kvm_vcpu *vcpu) {}
> +static inline void vcpu_reset_cptr(struct kvm_vcpu *vcpu) {}
> +
> +static inline bool kvm_vcpu_vfp_isdirty(struct kvm_vcpu *vcpu)
> +{
> +	return false;
> +}
> +
>  #endif /* __ARM64_KVM_EMULATE_H__ */
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 4562459..e16fd39 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -248,6 +248,7 @@ static inline void kvm_arch_hardware_unsetup(void) {}
>  static inline void kvm_arch_sync_events(struct kvm *kvm) {}
>  static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
>  static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
> +static inline void kvm_restore_host_vfp_state(struct kvm_vcpu *vcpu) {}
>  
>  void kvm_arm_init_debug(void);
>  void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
> -- 
> 1.9.1
> 

Thanks,
-Christoffer