Subject: [PATCH v4 2/3] KVM/arm/arm64: enable enhanced armv7 fp/simd lazy switch

From: [email-masked] (Mario Smarduch)
Date: Thu, 03 Dec 2015 17:07:55 -0800

On 12/3/2015 7:58 AM, Marc Zyngier wrote:
> On 14/11/15 22:12, Mario Smarduch wrote:
>> This patch tracks armv7 fp/simd hardware state with a vcpu lazy flag.
>> On vcpu_load saves host fpexc and enables FP access, and later enables fp/simd
>> trapping if lazy flag is not set. On first fp/simd access trap to handler 
>> to save host and restore guest context, disable trapping and set vcpu lazy 
>> flag. On vcpu_put if flag is set save guest and restore host context and 
>> always restore host fpexc.
>>
>> Signed-off-by: Mario Smarduch <[email-masked]>
>> ---
>>  arch/arm/include/asm/kvm_host.h   | 33 ++++++++++++++++++++++
>>  arch/arm/kvm/arm.c                | 12 ++++++++
>>  arch/arm/kvm/interrupts.S         | 58 +++++++++++++++++++++++----------------
>>  arch/arm/kvm/interrupts_head.S    | 26 +++++++++++++-----
>>  arch/arm64/include/asm/kvm_host.h |  6 ++++
>>  5 files changed, 104 insertions(+), 31 deletions(-)
>>
>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
>> index f1bf551..8fc7a59 100644
>> --- a/arch/arm/include/asm/kvm_host.h
>> +++ b/arch/arm/include/asm/kvm_host.h
>> @@ -40,6 +40,38 @@
>>  
>>  #define KVM_MAX_VCPUS VGIC_V2_MAX_CPUS
>>  
>> +/*
>> + * Reads the host FPEXC register, saves it to vcpu context and enables the
>> + * FP/SIMD unit.
>> + */
>> +#ifdef CONFIG_VFPv3
>> +#define kvm_enable_fpexc(vcpu) {			\
>> +	u32 fpexc = 0;					\
>> +	asm volatile(					\
>> +	"mrc p10, 7, %0, cr8, cr0, 0\n"			\
>> +	"str %0, [%1]\n"				\
>> +	"orr %0, %0, #(1 << 30)\n"			\
>> +	"mcr p10, 7, %0, cr8, cr0, 0\n"			\
> 
> Don't you need an ISB here? 
Yes it does (B2.7.3) - was thinking something else but the manual is clear here.

> Also, it would be a lot nicer if this was a
> real function (possibly inlined). I don't see any real reason to make
> this a #define.
Had some trouble reconciling arm and arm64 compile making this
a function in kvm_host.h. I'll work to resolve it.

> 
> Also, you're preserving a lot of the host's FPEXC bits. Is that safe?
No it may not be, should just set the enable bit.
> 
>> +	: "+r" (fpexc)					\
>> +	: "r" (&vcpu->arch.host_fpexc)			\
>> +	);						\
>> +}
>> +#else
>> +#define kvm_enable_fpexc(vcpu)
>> +#endif
>> +
>> +/* Restores host FPEXC register */
>> +#ifdef CONFIG_VFPv3
>> +#define kvm_restore_host_fpexc(vcpu) {			\
>> +	asm volatile(					\
>> +		"mcr p10, 7, %0, cr8, cr0, 0\n"		\
>> +		: : "r" (vcpu->arch.host_fpexc)		\
>> +	);						\
>> +}
>> +#else
>> +#define kvm_restore_host_fpexc(vcpu)
>> +#endif
>> +
>>  u32 *kvm_vcpu_reg(struct kvm_vcpu *vcpu, u8 reg_num, u32 mode);
>>  int __attribute_const__ kvm_target_cpu(void);
>>  int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
>> @@ -227,6 +259,7 @@ 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..cfc348a 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -291,10 +291,22 @@ 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_fpexc(vcpu);
>>  }
>>  
>>  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>  {
>> +	/* If the fp/simd registers are dirty save guest, restore host. */
>> +	if (vcpu->arch.vfp_dirty) {
> 
> See my previous comment about the dirty state.
Yes that change seems to be working out fine.
> 
>> +		kvm_restore_host_vfp_state(vcpu);
>> +		vcpu->arch.vfp_dirty = 0;
>> +	}
>> +
>> +	/* 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/interrupts.S b/arch/arm/kvm/interrupts.S
>> index 900ef6d..1ddaa89 100644
>> --- a/arch/arm/kvm/interrupts.S
>> +++ b/arch/arm/kvm/interrupts.S
>> @@ -28,6 +28,26 @@
>>  #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, vcpu, #VCPU_VFP_GUEST
>> +	store_vfp_state r7
>> +
>> +	add     r7, vcpu, #VCPU_VFP_HOST
>> +	ldr     r7, [r7]
>> +	restore_vfp_state r7
>> +
>> +	pop     {r4-r7}
>> +#endif
>> +	bx      lr
>> +ENDPROC(kvm_restore_host_vfp_state)
>>  
> 
> Please don't mix things that are part of the HYP code and things that
> are not in the same file. This is just asking for trouble. If that's not
> mapped into HYP, put it in a separate file.
Will do.
> 
>>  __kvm_hyp_code_start:
>>  	.globl __kvm_hyp_code_start
>> @@ -116,22 +136,22 @@ ENTRY(__kvm_vcpu_run)
>>  	read_cp15_state store_to_vcpu = 0
>>  	write_cp15_state read_from_vcpu = 1
>>  
>> +	set_hcptr_bits set, r4, (HCPTR_TTA)
>>  	@ 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
>> +	@ fp/simd register file has already been accessed, so skip trap enable.
>> +	vfp_skip_if_dirty r7, skip_guest_vfp_trap
>> +	set_hcptr_bits orr, r4, (HCPTR_TCP(10) | HCPTR_TCP(11))
>> +skip_guest_vfp_trap:
>>  #endif
>> +	set_hcptr vmentry, r4
>>  
>>  	@ 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 +190,8 @@ __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
>> -
>> -after_vfp_restore:
>> -	@ Restore FPEXC_EN which we clobbered on entry
>> -	pop	{r2}
>> -	VFPFMXR FPEXC, r2
>> -#else
>> -after_vfp_restore:
>> -#endif
>> +	set_hcptr_bits clear, r4, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
>> +	set_hcptr vmexit, r4
>>  
>>  	@ Reset Hyp-role
>>  	configure_hyp_role vmexit
>> @@ -483,7 +488,12 @@ switch_to_guest_vfp:
>>  	push	{r3-r7}
>>  
>>  	@ NEON/VFP used.  Turn on VFP access.
>> -	set_hcptr vmtrap, (HCPTR_TCP(10) | HCPTR_TCP(11))
>> +	set_hcptr_bits clear, r4, (HCPTR_TCP(10) | HCPTR_TCP(11))
>> +	set_hcptr vmtrap, r4
>> +
>> +	@ set lazy mode flag, switch hardware context on vcpu_put
>> +	mov     r1, #1
>> +	strb    r1, [vcpu, #VCPU_VFP_DIRTY]
> 
> You are assuming that a boolean is a byte. That's wrong, and not endian
> safe. If you want to have such a flag in a structure, use a sized type
> (u8, u16, u32). But again, I think we should rely on the trap flags to
> make decisions outside of the HYP code.
Although this is gone now, thanks for the tip very helpful.
> Thanks,
> 	M.