Re: [PATCH v3 23/41] KVM: PPC: Book3S HV P9: Reduce mftb per guest entry/exit

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

Nicholas Piggin <npiggin@gmail.com> writes:

> mftb is serialising (dispatch next-to-complete) so it is heavy weight
> for a mfspr. Avoid reading it multiple times in the entry or exit paths.
> A small number of cycles delay to timers is tolerable.
>
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>

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

> ---
>  arch/powerpc/kvm/book3s_hv.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index c1965a9d8d00..6f3e3aed99aa 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3505,12 +3505,13 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
>  		host_dawrx1 = mfspr(SPRN_DAWRX1);
>  	}
>
> -	hdec = time_limit - mftb();
> +	tb = mftb();
> +	hdec = time_limit - tb;
>  	if (hdec < 0)
>  		return BOOK3S_INTERRUPT_HV_DECREMENTER;
>
>  	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))
> @@ -3703,7 +3704,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
>  	if (!(vcpu->arch.ctrl & 1))
>  		mtspr(SPRN_CTRLT, mfspr(SPRN_CTRLF) & ~1);
>
> -	mtspr(SPRN_DEC, vcpu->arch.dec_expires - mftb());
> +	mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
>
>  	if (kvmhv_on_pseries()) {
>  		/*
> @@ -3837,7 +3838,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
>  	vc->entry_exit_map = 0x101;
>  	vc->in_guest = 0;
>
> -	mtspr(SPRN_DEC, local_paca->kvm_hstate.dec_expires - mftb());
> +	mtspr(SPRN_DEC, local_paca->kvm_hstate.dec_expires - tb);
>  	mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
>
>  	kvmhv_load_host_pmu();