Subject: Re: [PATCH v2 17/37] KVM: PPC: Book3S HV P9: Move setting HDEC after switching to guest LPCR

From: Fabiano Rosas <[email-masked]>
Date: Fri, 26 Feb 2021 13:38:15 -0300

Nicholas Piggin <[email-masked]> writes:

> LPCR[HDICE]=0 suppresses hypervisor decrementer exceptions on some
> processors, so it must be enabled before HDEC is set.
>
> Rather than set it in the host LPCR then setting HDEC, move the HDEC
> update to after the guest MMU context (including LPCR) is loaded.
> There shouldn't be much concern with delaying HDEC by some 10s or 100s
> of nanoseconds by setting it a bit later.
>
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 24 ++++++++++--------------
>  1 file changed, 10 insertions(+), 14 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index d4770b222d7e..63cc92c45c5d 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3490,23 +3490,13 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
>  		host_dawrx1 = mfspr(SPRN_DAWRX1);
>  	}
>
> -	/*
> -	 * P8 and P9 suppress the HDEC exception when LPCR[HDICE] = 0,
> -	 * so set HDICE before writing HDEC.
> -	 */
> -	mtspr(SPRN_LPCR, kvm->arch.host_lpcr | LPCR_HDICE);
> -	isync();
> -
> -	hdec = time_limit - mftb();

Would it be possible to leave the mftb() in this patch and then replace
them all at once in patch 20/37 - "KVM: PPC: Book3S HV P9: Reduce mftb
per guest entry/exit"?

> -	if (hdec < 0) {
> -		mtspr(SPRN_LPCR, kvm->arch.host_lpcr);
> -		isync();
> +	tb = mftb();
> +	hdec = time_limit - tb;
> +	if (hdec < 0)
>  		return BOOK3S_INTERRUPT_HV_DECREMENTER;
> -	}
> -	mtspr(SPRN_HDEC, hdec);
>
>  	if (vc->tb_offset) {
> -		u64 new_tb = mftb() + vc->tb_offset;
> +		u64 new_tb = tb + vc->tb_offset;
>  		mtspr(SPRN_TBU40, new_tb);
>  		tb = mftb();
>  		if ((tb & 0xffffff) < (new_tb & 0xffffff))
> @@ -3549,6 +3539,12 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
>
>  	switch_mmu_to_guest_radix(kvm, vcpu, lpcr);
>
> +	/*
> +	 * P9 suppresses the HDEC exception when LPCR[HDICE] = 0,
> +	 * so set guest LPCR (with HDICE) before writing HDEC.
> +	 */
> +	mtspr(SPRN_HDEC, hdec);
> +
>  	mtspr(SPRN_SRR0, vcpu->arch.shregs.srr0);
>  	mtspr(SPRN_SRR1, vcpu->arch.shregs.srr1);