Hello,
On 18-Sep-15 03:05, Mario Smarduch wrote:
> Adds code to enable fp/simd lazy switch. On each entry check if fp/simd
> registers have been switched to guest, if no set the trap flag. On trap
> switch fp/simd registers and set vfp_lazy to true and disable trapping.
> When the vcpu is about to be put, then context switch fp/simd registers
> save guest and restore host and reset the vfp_lazy state to enable trapping
> again.
>
This description confused me a bit, since KVM on ARMv7 already exhibits
lazy switching behavior for VFP. Should the description highlight the
intended improvement in behavior?
If I understand correctly, instead of restoring the host state on every
exit, you postpone it until the task actually gets rescheduled, right?
> Signed-off-by: Mario Smarduch <m.smarduch@xxxxxxxxxxx>
> ---
> arch/arm/kvm/arm.c | 17 +++++++++++++++++
> arch/arm/kvm/interrupts.S | 40 +++++++++++++++++++++++++++++-----------
> 2 files changed, 46 insertions(+), 11 deletions(-)
>
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index ce404a5..0acbb69 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.
> + *
> + */
> +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);
> + vcpu->arch.vfp_lazy = 0;
> + }
> +#endif
> +}
>
> /**
> * 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 */
> + 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..a47acc1 100644
> --- a/arch/arm/kvm/interrupts.S
> +++ b/arch/arm/kvm/interrupts.S
> @@ -96,6 +96,24 @@ 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.
> + *
> + */
> +ENTRY(__kvm_restore_host_vfp_state)
> + 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
> +
> + pop {r3-r7}
> + bx lr
> +ENDPROC(__kvm_restore_host_vfp_state)
>
> /********************************************************************
> * Hypervisor world-switch code
> @@ -131,7 +149,14 @@ ENTRY(__kvm_vcpu_run)
>
> @ Trap coprocessor CRx accesses
> set_hstr vmentry
> +
> + ldr r1, [vcpu, #VCPU_VFP_LAZY]
> + cmp r1, #1
> + beq skip_guest_vfp_trap
> +
> set_hcptr vmentry, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
> +skip_guest_vfp_trap:
I believe that HCPTR_TTA is not part of the floating point extensions.
> +
> set_hdcr vmentry
>
> @ Write configured ID register into MIDR alias
> @@ -170,22 +195,12 @@ __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))
If you don't use the functionality of the macro to branch on change,
then maybe the functionality should also be removed from the macro.
>
> #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
>
> @ Reset Hyp-role
> @@ -485,6 +500,9 @@ switch_to_guest_vfp:
> @ NEON/VFP used. Turn on VFP access.
> set_hcptr vmtrap, (HCPTR_TCP(10) | HCPTR_TCP(11))
>
> + 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]
>
--
Antonios Motakis
Virtualization Engineer
Huawei Technologies Duesseldorf GmbH
European Research Center
Riesstrasse 25, 80992 München
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