Re: [PATCH v10 05/18] KVM: arm64: Convert lazy FPSIMD context switch trap to C

["Alex Bennée" <alex.bennee@linaro.org>]

Dave Martin <Dave.Martin@arm.com> writes:

> To make the lazy FPSIMD context switch trap code easier to hack on,
> this patch converts it to C.
>
> This is not amazingly efficient, but the trap should typically only
> be taken once per host context switch.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  arch/arm64/kvm/hyp/entry.S  | 57 +++++++++++++++++----------------------------
>  arch/arm64/kvm/hyp/switch.c | 24 +++++++++++++++++++
>  2 files changed, 46 insertions(+), 35 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> index e41a161..40f349b 100644
> --- a/arch/arm64/kvm/hyp/entry.S
> +++ b/arch/arm64/kvm/hyp/entry.S
> @@ -172,40 +172,27 @@ ENTRY(__fpsimd_guest_restore)
>  	// x1: vcpu
>  	// x2-x29,lr: vcpu regs
>  	// vcpu x0-x1 on the stack
> -	stp	x2, x3, [sp, #-16]!
> -	stp	x4, lr, [sp, #-16]!
> -
> -alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
> -	mrs	x2, cptr_el2
> -	bic	x2, x2, #CPTR_EL2_TFP
> -	msr	cptr_el2, x2
> -alternative_else
> -	mrs	x2, cpacr_el1
> -	orr	x2, x2, #CPACR_EL1_FPEN
> -	msr	cpacr_el1, x2
> -alternative_endif
> -	isb
> -
> -	mov	x3, x1
> -
> -	ldr	x0, [x3, #VCPU_HOST_CONTEXT]
> -	kern_hyp_va x0
> -	add	x0, x0, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> -	bl	__fpsimd_save_state
> -
> -	add	x2, x3, #VCPU_CONTEXT
> -	add	x0, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> -	bl	__fpsimd_restore_state
> -
> -	// Skip restoring fpexc32 for AArch64 guests
> -	mrs	x1, hcr_el2
> -	tbnz	x1, #HCR_RW_SHIFT, 1f
> -	ldr	x4, [x3, #VCPU_FPEXC32_EL2]
> -	msr	fpexc32_el2, x4
> -1:
> -	ldp	x4, lr, [sp], #16
> -	ldp	x2, x3, [sp], #16
> -	ldp	x0, x1, [sp], #16
> -
> +	stp	x2, x3, [sp, #-144]!
> +	stp	x4, x5, [sp, #16]
> +	stp	x6, x7, [sp, #32]
> +	stp	x8, x9, [sp, #48]
> +	stp	x10, x11, [sp, #64]
> +	stp	x12, x13, [sp, #80]
> +	stp	x14, x15, [sp, #96]
> +	stp	x16, x17, [sp, #112]
> +	stp	x18, lr, [sp, #128]
> +
> +	bl	__hyp_switch_fpsimd
> +
> +	ldp	x4, x5, [sp, #16]
> +	ldp	x6, x7, [sp, #32]
> +	ldp	x8, x9, [sp, #48]
> +	ldp	x10, x11, [sp, #64]
> +	ldp	x12, x13, [sp, #80]
> +	ldp	x14, x15, [sp, #96]
> +	ldp	x16, x17, [sp, #112]
> +	ldp	x18, lr, [sp, #128]
> +	ldp	x0, x1, [sp, #144]
> +	ldp	x2, x3, [sp], #160
>  	eret
>  ENDPROC(__fpsimd_guest_restore)
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index d964523..c0796c4 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -318,6 +318,30 @@ static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
>  	}
>  }
>
> +void __hyp_text __hyp_switch_fpsimd(u64 esr __always_unused,
> +				    struct kvm_vcpu *vcpu)
> +{
> +	kvm_cpu_context_t *host_ctxt;
> +
> +	if (has_vhe())
> +		write_sysreg(read_sysreg(cpacr_el1) | CPACR_EL1_FPEN,
> +			     cpacr_el1);
> +	else
> +		write_sysreg(read_sysreg(cptr_el2) & ~(u64)CPTR_EL2_TFP,
> +			     cptr_el2);

Is there no way to do alternative() in C or does it always come down to
different inline asms?

Anyway:

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>


> +
> +	isb();
> +
> +	host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> +	__fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> +	__fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
> +
> +	/* Skip restoring fpexc32 for AArch64 guests */
> +	if (!(read_sysreg(hcr_el2) & HCR_RW))
> +		write_sysreg(vcpu->arch.ctxt.sys_regs[FPEXC32_EL2],
> +			     fpexc32_el2);
> +}
> +
>  /*
>   * Return true when we were able to fixup the guest exit and should return to
>   * the guest, false when we should restore the host state and return to the


--
Alex Bennée

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