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

[Mario Smarduch <m.smarduch@samsung.com>]

On 11/6/2015 3:37 AM, Christoffer Dall wrote:
> On Thu, Nov 05, 2015 at 04:23:41PM -0800, Mario Smarduch wrote:
>>
>>
>> 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 <m.smarduch@samsung.com>
>>>> ---
>>>>  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().
> 
> If you're preempted, vcpu_put will be called.  See kvm_preempt_ops in
> virt/kvm/kvm_main.c.
Yes we can cleanup in vcpu_put what we do in vcpu_load.
> 
>>
>>>
>>>> +
>>>>  	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.
> 
> here, you do:
> 
> 	ldr r4, HCPTR_TTA
> 	vfp_skip_if_dirty skip_vfp_trap
> 	orr r4, r4, #(HCPTR_TCP(10) | HCPTR_TCP(11))
> skip_vfp_trap:
> 	set_hcptr vmentry, r4
> 
> if that works with the necessary rework of set_hcptr to take a register,
> if the orr can be encoded propertly etc.  Maybe it's not worth it, it
> just feels weird to touch this registers twice. 
Yes definitely,  thanks for spelling out the details. A simple or looks quite
different in assembler - we hates it :)

Thanks.
> Perhaps the nicer fix
> is to just rename/refactor set_hcptr to be two functions, set_hcptr_bits
> and clear_hcptr_bits.
> 
> Thanks,
> -Christoffer
>