Subject: [PATCH 3/3] KVM/arm64: enable enhanced armv8 fp/simd lazy switch

From: [email-masked] (Mario Smarduch)
Date: Thu, 05 Nov 2015 16:57:12 -0800

On 11/5/2015 7:02 AM, Christoffer Dall wrote:
> On Fri, Oct 30, 2015 at 02:56:33PM -0700, Mario Smarduch wrote:
>> This patch enables arm64 lazy fp/simd switch, similar to arm described in
>> second patch. Change from previous version - restore function is moved to
>> host. 
>>
>> Signed-off-by: Mario Smarduch <[email-masked]>
>> ---
>>  arch/arm64/include/asm/kvm_host.h |  2 +-
>>  arch/arm64/kernel/asm-offsets.c   |  1 +
>>  arch/arm64/kvm/hyp.S              | 37 +++++++++++++++++++++++++++++++------
>>  3 files changed, 33 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 26a2347..dcecf92 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -251,11 +251,11 @@ 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);
>>  void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
>>  void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
>> +void kvm_restore_host_vfp_state(struct kvm_vcpu *vcpu);
>>  
>>  #endif /* __ARM64_KVM_HOST_H__ */
>> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
>> index 8d89cf8..c9c5242 100644
>> --- a/arch/arm64/kernel/asm-offsets.c
>> +++ b/arch/arm64/kernel/asm-offsets.c
>> @@ -124,6 +124,7 @@ int main(void)
>>    DEFINE(VCPU_HCR_EL2,		offsetof(struct kvm_vcpu, arch.hcr_el2));
>>    DEFINE(VCPU_MDCR_EL2,	offsetof(struct kvm_vcpu, arch.mdcr_el2));
>>    DEFINE(VCPU_IRQ_LINES,	offsetof(struct kvm_vcpu, arch.irq_lines));
>> +  DEFINE(VCPU_VFP_DIRTY,	offsetof(struct kvm_vcpu, arch.vfp_dirty));
>>    DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
>>    DEFINE(VCPU_HOST_DEBUG_STATE, offsetof(struct kvm_vcpu, arch.host_debug_state));
>>    DEFINE(VCPU_TIMER_CNTV_CTL,	offsetof(struct kvm_vcpu, arch.timer_cpu.cntv_ctl));
>> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
>> index e583613..ed2c4cf 100644
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -36,6 +36,28 @@
>>  #define CPU_SYSREG_OFFSET(x)	(CPU_SYSREGS + 8*x)
>>  
>>  	.text
>> +
>> +/**
>> + * void kvm_restore_host_vfp_state(struct vcpu *vcpu) - Executes lazy
>> + *	fp/simd switch, saves the guest, restores host. Called from host
>> + *	mode, placed outside of hyp section.
> 
> same comments on style as previous patch
Got it.
> 
>> + */
>> +ENTRY(kvm_restore_host_vfp_state)
>> +	push    xzr, lr
>> +
>> +	add     x2, x0, #VCPU_CONTEXT
>> +	mov     w3, #0
>> +	strb    w3, [x0, #VCPU_VFP_DIRTY]
> 
> I've been discussing with myself if it would make more sense to clear
> the dirty flag in the C-code...
Since all the work is done here I placed it here.
> 
>> +
>> +	bl __save_fpsimd
>> +
>> +	ldr     x2, [x0, #VCPU_HOST_CONTEXT]
>> +	bl __restore_fpsimd
>> +
>> +	pop     xzr, lr
>> +	ret
>> +ENDPROC(kvm_restore_host_vfp_state)
>> +
>>  	.pushsection	.hyp.text, "ax"
>>  	.align	PAGE_SHIFT
>>  
>> @@ -482,7 +504,11 @@
>>  99:
>>  	msr     hcr_el2, x2
>>  	mov	x2, #CPTR_EL2_TTA
>> +
>> +	ldrb	w3, [x0, #VCPU_VFP_DIRTY]
>> +	tbnz    w3, #0, 98f
>>  	orr     x2, x2, #CPTR_EL2_TFP
>> +98:
> 
> mmm, don't you need to only set the fpexc32 when you're actually going
> to trap the guest accesses?

My understanding is you always need to set enable in fpexec32 for 32 bit guests,
otherwise EL1 would get the trap instead of EL2. Not sure if that's the point
you're making.

> 
> also, you can consider only setting this in vcpu_load (jumping quickly
> to EL2 to do so) if we're running a 32-bit guest.  Probably worth
> measuring the difference between the extra EL2 jump on vcpu_load
> compared to hitting this register on every entry/exit.

Sure, makes sense since this is a hot code path.
> 
> Code-wise, it will be nicer to do it on vcpu_load.
> 
>>  	msr	cptr_el2, x2
>>  
>>  	mov	x2, #(1 << 15)	// Trap CP15 Cr=15
>> @@ -669,14 +695,12 @@ __restore_debug:
>>  	ret
>>  
>>  __save_fpsimd:
>> -	skip_fpsimd_state x3, 1f
>>  	save_fpsimd
>> -1:	ret
>> +	ret
>>  
>>  __restore_fpsimd:
>> -	skip_fpsimd_state x3, 1f
>>  	restore_fpsimd
>> -1:	ret
>> +	ret
>>  
>>  switch_to_guest_fpsimd:
>>  	push	x4, lr
>> @@ -688,6 +712,9 @@ switch_to_guest_fpsimd:
>>  
>>  	mrs	x0, tpidr_el2
>>  
>> +	mov     w2, #1
>> +	strb    w2, [x0, #VCPU_VFP_DIRTY]
> 
> hmm, just noticing this.  Are you not writing a 32-bit value to a
> potentially 8-bit field (ignoring padding in the struct), as the dirty
> flag is declared a bool.

>>From the manuals byte stores require a word size registers on arm64/arm32.
The interconnect probably drops the remaining bytes.
Also double checked and the compiler uses same instructions.

> 
> Are you also doing this on the 32-bit side?
strb is used on both v7/v8
> 
>> +
>>  	ldr	x2, [x0, #VCPU_HOST_CONTEXT]
>>  	kern_hyp_va x2
>>  	bl __save_fpsimd
>> @@ -763,7 +790,6 @@ __kvm_vcpu_return:
>>  	add	x2, x0, #VCPU_CONTEXT
>>  
>>  	save_guest_regs
>> -	bl __save_fpsimd
>>  	bl __save_sysregs
>>  
>>  	skip_debug_state x3, 1f
>> @@ -784,7 +810,6 @@ __kvm_vcpu_return:
>>  	kern_hyp_va x2
>>  
>>  	bl __restore_sysregs
>> -	bl __restore_fpsimd
>>  	/* Clear FPSIMD and Trace trapping */
>>  	msr     cptr_el2, xzr
>>  
>> -- 
>> 1.9.1
>>
> 
> Thanks,
> -Christoffer
>