Re: [PATCH v3 20/41] KVM: PPC: Book3S HV P9: Move setting HDEC after switching to guest LPCR

[Fabiano Rosas <farosas@linux.ibm.com>]

Nicholas Piggin <npiggin@gmail.com> 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 <npiggin@gmail.com>

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>

> ---
>  arch/powerpc/kvm/book3s_hv.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 1f2ba8955c6a..ffde1917ab68 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3505,20 +3505,9 @@ 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();
> -	if (hdec < 0) {
> -		mtspr(SPRN_LPCR, kvm->arch.host_lpcr);
> -		isync();
> +	if (hdec < 0)
>  		return BOOK3S_INTERRUPT_HV_DECREMENTER;
> -	}
> -	mtspr(SPRN_HDEC, hdec);
>
>  	if (vc->tb_offset) {
>  		u64 new_tb = mftb() + vc->tb_offset;
> @@ -3564,6 +3553,12 @@ 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);