Subject: [PATCH v2 2/2] KVM/arm: enable enhanced armv7 fp/simd lazy switch

From: [email-masked] (Mario Smarduch)
Date: Mon, 19 Oct 2015 16:25:04 -0700

On 10/19/2015 3:14 AM, Christoffer Dall wrote:
> On Sat, Sep 26, 2015 at 04:43:29PM -0700, Mario Smarduch wrote:
>> This patch enhances current lazy vfp/simd hardware switch. In addition to
>> current lazy switch, it tracks vfp/simd hardware state with a vcpu 
>> lazy flag. 
>>
>> vcpu lazy flag is set on guest access and trap to vfp/simd hardware switch 
>> handler. On vm-enter if lazy flag is set skip trap enable and saving 
>> host fpexc. On vm-exit if flag is set skip hardware context switch
>> and return to host with guest context.
>>
>> On vcpu_put check if vcpu lazy flag is set, and execute a hardware context 
>> switch to restore host.
>>
>> Signed-off-by: Mario Smarduch <[email-masked]>
>> ---
>>  arch/arm/include/asm/kvm_asm.h |  1 +
>>  arch/arm/kvm/arm.c             | 17 ++++++++++++
>>  arch/arm/kvm/interrupts.S      | 60 +++++++++++++++++++++++++++++++-----------
>>  arch/arm/kvm/interrupts_head.S | 12 ++++++---
>>  4 files changed, 71 insertions(+), 19 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_asm.h b/arch/arm/include/asm/kvm_asm.h
>> index 194c91b..4b45d79 100644
>> --- a/arch/arm/include/asm/kvm_asm.h
>> +++ b/arch/arm/include/asm/kvm_asm.h
>> @@ -97,6 +97,7 @@ extern char __kvm_hyp_code_end[];
>>  extern void __kvm_flush_vm_context(void);
>>  extern void __kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa);
>>  extern void __kvm_tlb_flush_vmid(struct kvm *kvm);
>> +extern void __kvm_restore_host_vfp_state(struct kvm_vcpu *vcpu);
>>  
>>  extern int __kvm_vcpu_run(struct kvm_vcpu *vcpu);
>>  #endif
>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>> index ce404a5..79f49c7 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -105,6 +105,20 @@ void kvm_arch_check_processor_compat(void *rtn)
>>  	*(int *)rtn = 0;
>>  }
>>  
>> +/**
>> + * kvm_switch_fp_regs() - switch guest/host VFP/SIMD registers
>> + * @vcpu:	pointer to vcpu structure.
>> + *
> 
> nit: stray blank line
ok
> 
>> + */
>> +static void kvm_switch_fp_regs(struct kvm_vcpu *vcpu)
>> +{
>> +#ifdef CONFIG_ARM
>> +	if (vcpu->arch.vfp_lazy == 1) {
>> +		kvm_call_hyp(__kvm_restore_host_vfp_state, vcpu);
> 
> why do you have to do this in HYP mode ?
 Calling it directly works fine. I moved the function outside hyp start/end
range in interrupts.S. Not thinking outside the box, just thought let them all
be hyp calls.

> 
>> +		vcpu->arch.vfp_lazy = 0;
>> +	}
>> +#endif
> 
> we've tried to put stuff like this in header files to avoid the ifdefs
> so far.  Could that be done here as well?

That was a to do, but didn't get around to it.
> 
>> +}
>>  
>>  /**
>>   * kvm_arch_init_vm - initializes a VM data structure
>> @@ -295,6 +309,9 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>  
>>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>  {
>> +	/* Check if Guest accessed VFP registers */
> 
> misleading comment: this function does more than checking
Yep sure does, will change.
> 
>> +	kvm_switch_fp_regs(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/interrupts.S b/arch/arm/kvm/interrupts.S
>> index 900ef6d..6d98232 100644
>> --- a/arch/arm/kvm/interrupts.S
>> +++ b/arch/arm/kvm/interrupts.S
>> @@ -96,6 +96,29 @@ ENTRY(__kvm_flush_vm_context)
>>  	bx	lr
>>  ENDPROC(__kvm_flush_vm_context)
>>  
>> +/**
>> + * void __kvm_restore_host_vfp_state(struct vcpu *vcpu) - Executes a lazy
>> + *     fp/simd switch, saves the guest, restores host.
>> + *
> 
> nit: stray blank line
ok.
> 
>> + */
>> +ENTRY(__kvm_restore_host_vfp_state)
>> +#ifdef CONFIG_VFPv3
>> +	push	{r3-r7}
>> +
>> +	add	r7, r0, #VCPU_VFP_GUEST
>> +	store_vfp_state r7
>> +
>> +	add	r7, r0, #VCPU_VFP_HOST
>> +	ldr	r7, [r7]
>> +	restore_vfp_state r7
>> +
>> +	ldr	r3, [vcpu, #VCPU_VFP_FPEXC]
> 
> either use r0 or vcpu throughout this function please
Yeah that's bad - in the same function to
> 
>> +	VFPFMXR FPEXC, r3
>> +
>> +	pop	{r3-r7}
>> +#endif
>> +	bx	lr
>> +ENDPROC(__kvm_restore_host_vfp_state)
>>  
>>  /********************************************************************
>>   *  Hypervisor world-switch code
>> @@ -119,11 +142,15 @@ 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
>> +	@ r7 must be preserved until next vfp lazy check
> 
> I don't understand this comment
> 
>> +	vfp_inlazy_mode r7, skip_save_host_fpexc
>> +
>>  	@ Set FPEXC_EN so the guest doesn't trap floating point instructions
>>  	VFPFMRX r2, FPEXC		@ VMRS
>> -	push	{r2}
>> +	str     r2, [vcpu, #VCPU_VFP_FPEXC]
>>  	orr	r2, r2, #FPEXC_EN
>>  	VFPFMXR FPEXC, r2		@ VMSR
>> +skip_save_host_fpexc:
>>  #endif
>>  
>>  	@ Configure Hyp-role
>> @@ -131,7 +158,14 @@ 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)
>> +
>> +	@ check if vfp_lazy flag set
>> +	cmp     r7, #1
> 
> if you meant that you need to preserve r7 down to here, could you
> instead just move the VFP logic above down here and do the whole VFP
> logic in one coherent block?

I reworked the code both fpexc save and trap enable are handled at once.
> 
>> +	beq     skip_guest_vfp_trap
>> +	set_hcptr vmentry, (HCPTR_TCP(10) | HCPTR_TCP(11))
>> +skip_guest_vfp_trap:
>> +
>>  	set_hdcr vmentry
>>  
>>  	@ Write configured ID register into MIDR alias
>> @@ -170,22 +204,14 @@ __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}
>> +	vfp_inlazy_mode r2, skip_restore_host_fpexc
>> +	@ If vfp_lazy is not set, restore FPEXC_EN which we clobbered on entry
>> +	ldr     r2, [vcpu, #VCPU_VFP_FPEXC]
> 
> so we do this restore if, since we scheduled this VCPU thread, the guest
> has not touched any VFP regs, is that correct?
That's right.
> 
> Did you measure how often we schedule the guest and run it until we
> schedule another process without the guest touching any VFP regs?  I'm
> just wondering if this complexity is worth it, or if we should just
> switch the VFP regs on vcpu_load/vcpu_put instead?

The loads I've been running mix of fp operations and lmbench mmu - shows huge
decrease of fp save/restore like from ~30-50%, down to ~2%. What I did is
measured all exits and fp/save restore for both scenarios. So yes it does make a
difference. Of course will depend on the load, but should be never be worse then
now.
> 
> Also, what do other architectures do here?

x86 does a similar thing in it's kvm_arch_vcpu_put().

> 
>>  	VFPFMXR FPEXC, r2
>> -#else
>> -after_vfp_restore:
>> +skip_restore_host_fpexc:
>>  #endif
>>  
>>  	@ Reset Hyp-role
>> @@ -485,6 +511,10 @@ 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
>> +	str     r1, [vcpu, #VCPU_VFP_LAZY]
>> +
>>  	@ 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 702740d..4561171 100644
>> --- a/arch/arm/kvm/interrupts_head.S
>> +++ b/arch/arm/kvm/interrupts_head.S
>> @@ -594,7 +594,7 @@ ARM_BE8(rev	r6, r6  )
>>   * 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
>> @@ -609,13 +609,17 @@ ARM_BE8(rev	r6, r6  )
>>  	beq	1f
>>  	.endif
>>  	isb
>> -	.if \label != none
>> -	b	\label
>> -	.endif
>>  1:
>>  	.endif
>>  .endm
>>  
>> +/* Checks if VFP/SIMD lazy flag is set, if it is branch to label. */
> 
> I don't easily understand the semantics of the lazy flag.  When set,
> does it mean we've switched the hardware to the guest state?
> 
>> +.macro vfp_inlazy_mode, reg, label
>> +	ldr	\reg, [vcpu, #VCPU_VFP_LAZY]
>> +	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
>> -- 
>> 1.9.1
>>
> 
> Thanks!
> -Christoffer
>