Subject: [PATCH v3 2/3] KVM/arm/arm64: enable enhanced armv7 fp/simd lazy switch

From: [email-masked] (Mario Smarduch)
Date: Thu, 05 Nov 2015 16:23:41 -0800

On 11/5/2015 6:48 AM, Christoffer Dall wrote:
> On Fri, Oct 30, 2015 at 02:56:32PM -0700, Mario Smarduch wrote:
>> This patch tracks vfp/simd hardware state with a vcpu lazy flag. vCPU lazy 
>> flag is set on guest access and traps to vfp/simd hardware switch handler. On 
>> vm-enter if lazy flag is set skip trap enable and save host fpexc. On 
>> vm-exit if flag is set skip hardware context switch and return to host with 
>> guest context. In vcpu_put check if vcpu lazy flag is set, and execute a 
>> hardware context switch to restore host.
>>
>> Also some arm64 field and empty function are added to compile for arm64.
>>
>> Signed-off-by: Mario Smarduch <[email-masked]>
>> ---
>>  arch/arm/include/asm/kvm_host.h   |  1 +
>>  arch/arm/kvm/arm.c                |  6 ++++
>>  arch/arm/kvm/interrupts.S         | 60 ++++++++++++++++++++++++++++-----------
>>  arch/arm/kvm/interrupts_head.S    | 14 +++++----
>>  arch/arm64/include/asm/kvm_host.h |  4 +++
>>  5 files changed, 63 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index f1bf551..a9e86e0 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/arm.c b/arch/arm/kvm/arm.c
>> index dc017ad..11a56fe 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -296,6 +296,12 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>  {
>>  	/*
>> +	 * If fp/simd registers are dirty save guest, restore host before
> 
> If the fp/simd registers are dirty, then restore the host state before
I'd drop 'releasing the cpu', the vcpu thread may be returning to
user mode.
> 
>> +	 * releasing the cpu.
>> +	 */
>> +	if (vcpu->arch.vfp_dirty)
>> +		kvm_restore_host_vfp_state(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
>>  	 * optimized make_all_cpus_request path.
>> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
>> index 900ef6d..ca25314 100644
>> --- a/arch/arm/kvm/interrupts.S
>> +++ b/arch/arm/kvm/interrupts.S
>> @@ -28,6 +28,32 @@
>>  #include "interrupts_head.S"
>>  
>>  	.text
>> +/**
>> + * void kvm_restore_host_vfp_state(struct vcpu *vcpu) - Executes lazy
> 
> nit: Can you move the multi-line description of the function into a
> separate paragraph?
Sure.
> 
>> + * 	fp/simd switch, saves the guest, restores host. Called from host
>> + *	mode, placed outside of hyp region start/end.
> 
> Put the description in a separate paragraph and get rid of the "executes
> lazy fp/simd swithch" part, that doesn't help understanding.  Just say
> that this funciton restores the host state.
Sure.
> 
>> + */
>> +ENTRY(kvm_restore_host_vfp_state)
>> +#ifdef CONFIG_VFPv3
>> +	push    {r4-r7}
>> +
>> +	add     r7, vcpu, #VCPU_VFP_GUEST
>> +	store_vfp_state r7
>> +
>> +	add     r7, vcpu, #VCPU_VFP_HOST
>> +	ldr     r7, [r7]
>> +	restore_vfp_state r7
>> +
>> +	ldr     r3, [vcpu, #VCPU_VFP_HOST_FPEXC]
>> +	VFPFMXR FPEXC, r3
>> +
>> +	mov     r3, #0
>> +	strb    r3, [vcpu, #VCPU_VFP_DIRTY]
>> +
>> +	pop     {r4-r7}
>> +#endif
>> +	bx      lr
>> +ENDPROC(kvm_restore_host_vfp_state)
>>  
>>  __kvm_hyp_code_start:
>>  	.globl __kvm_hyp_code_start
>> @@ -119,11 +145,16 @@ ENTRY(__kvm_vcpu_run)
>>  	@ 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
>> +	@ fp/simd register file has already been accessed, so skip host fpexc
>> +	@ save and access trap enable.
>> +	vfp_inlazy_mode r7, skip_guest_vfp_trap
> 
> So, why do we need to touch this register at all on every CPU exit?
> 
> Is it not true that we can only be in one of two state:
>  1) The register file is not dirty (not touched by the guest) and we
>     should trap
>  2) The register file is dirty, and we should not trap to EL2?
> 
> Only in the first case do we need to set the FPEXC, and couldn't we just
> do that on vcpu_load and git rid of all this?  (except HCPTR_TCP which
> we still need to adjust).

I'm trying to think what happens if you're preempted after you saved
the FPEXC and set the FPEXC_EN bit in kvm_arch_vcpu_load(). Could some
thread pick up a bad FPEXC? May be possible to undo in the vcpu_put().

> 
>> +
>>  	VFPFMRX r2, FPEXC		@ VMRS
>> -	push	{r2}
>> +	str     r2, [vcpu, #VCPU_VFP_HOST_FPEXC]
>>  	orr	r2, r2, #FPEXC_EN
>>  	VFPFMXR FPEXC, r2		@ VMSR
>> +	set_hcptr vmentry, (HCPTR_TCP(10) | HCPTR_TCP(11))
>> +skip_guest_vfp_trap:
>>  #endif
>>  
>>  	@ Configure Hyp-role
>> @@ -131,7 +162,7 @@ ENTRY(__kvm_vcpu_run)
>>  
>>  	@ Trap coprocessor CRx accesses
>>  	set_hstr vmentry
>> -	set_hcptr vmentry, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
>> +	set_hcptr vmentry, (HCPTR_TTA)
> 
> based on the above I think you can rework this to set the mask based on
> the dirty flag and only hit the HCPTR once.

Not sure how to do this, tracing always needs to be enabled, and it's
independent of FP dirty state.

> 
>>  	set_hdcr vmentry
>>  
>>  	@ Write configured ID register into MIDR alias
>> @@ -170,22 +201,15 @@ __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
>> +	set_hcptr vmexit, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
>>  
>>  #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
>> -
>> -after_vfp_restore:
>> -	@ Restore FPEXC_EN which we clobbered on entry
>> -	pop	{r2}
>> +	@ If fp/simd not dirty, restore FPEXC which we clobbered on entry.
>> +	@ Otherwise return with guest FPEXC, later saved in vcpu_put.
>> +	vfp_inlazy_mode r2, skip_restore_host_fpexc
>> +	ldr     r2, [vcpu, #VCPU_VFP_HOST_FPEXC]
> 
> I think you should always just restore in this in vcpu_put instead of
> hitting on this all the time.
True, I was thinking of stack unwinding from older code.
> 
>>  	VFPFMXR FPEXC, r2
>> -#else
>> -after_vfp_restore:
>> +skip_restore_host_fpexc:
>>  #endif
>>  
>>  	@ Reset Hyp-role
>> @@ -485,6 +509,10 @@ switch_to_guest_vfp:
>>  	@ NEON/VFP used.  Turn on VFP access.
>>  	set_hcptr vmtrap, (HCPTR_TCP(10) | HCPTR_TCP(11))
>>  
>> +	@ set lazy mode flag, switch hardware context on vcpu_put
>> +	mov     r1, #1
>> +	strb    r1, [vcpu, #VCPU_VFP_DIRTY]
>> +
>>  	@ Switch VFP/NEON hardware state to the guest's
>>  	add	r7, r0, #VCPU_VFP_HOST
>>  	ldr	r7, [r7]
>> diff --git a/arch/arm/kvm/interrupts_head.S b/arch/arm/kvm/interrupts_head.S
>> index 51a5950..34e71ee 100644
>> --- a/arch/arm/kvm/interrupts_head.S
>> +++ b/arch/arm/kvm/interrupts_head.S
>> @@ -593,10 +593,8 @@ 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
>> +.macro set_hcptr operation, mask
>>  	mrc	p15, 4, r2, c1, c1, 2
>>  	ldr	r3, =\mask
>>  	.if \operation == vmentry
>> @@ -611,13 +609,17 @@ ARM_BE8(rev	r6, r6  )
>>  	beq	1f
>>  	.endif
>>  	isb
>> -	.if \label != none
>> -	b	\label
>> -	.endif
>>  1:
>>  	.endif
>>  .endm
>>  
>> +/* Checks if VFP/SIMD dirty flag is set, if it is branch to label. */
>> +.macro vfp_inlazy_mode, reg, label
> 
> Again, I don't find this "in lazy mode" to mean that we've now switched
> to the guest's VFP state intuitive or logical.
> 
> How about vfp_skip_if_dirty ?
Yes sounds good.

> 
>> +	ldr	\reg, [vcpu, #VCPU_VFP_DIRTY]
>> +	cmp	\reg, #1
>> +	beq	\label
>> +.endm
>> +
>>  /* Configures the HDCR (Hyp Debug Configuration Register) on entry/return
>>   * (hardware reset value is 0) */
>>  .macro set_hdcr operation
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 4562459..26a2347 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -157,6 +157,9 @@ struct kvm_vcpu_arch {
>>  	/* Interrupt related fields */
>>  	u64 irq_lines;		/* IRQ and FIQ levels */
>>  
>> +	/* fp/simd dirty flag true if guest accessed register file */
>> +	bool    vfp_dirty;
>> +
>>  	/* Cache some mmu pages needed inside spinlock regions */
>>  	struct kvm_mmu_memory_cache mmu_page_cache;
>>  
>> @@ -248,6 +251,7 @@ 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
>>