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

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/03/2021 02:06, Nicholas Piggin wrote:
> LPCR[HDICE]=0 suppresses hypervisor decrementer exceptions on some
> processors, so it must be enabled before HDEC is set.

Educating myself - is not it a processor bug when it does not suppress 
hdec exceptions with HDICE=0?

Also, why do we want to enable interrupts before writing HDEC? Enabling 
it may cause an interrupt right away a

Anyway whatever the answers are, this is not changed by this patch and 
the change makes sense so

Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>


> 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>
> ---
>   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);
>   
> 

-- 
Alexey