[PATCH v2 2/2] KVM/arm: enable enhanced armv7 fp/simd lazy switch
Mario Smarduch
m.smarduch at samsung.com
Mon Oct 19 16:25:04 PDT 2015
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 <m.smarduch at samsung.com>
>> ---
>> 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
>
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