Re: [PATCH v3 25/41] KVM: PPC: Book3S HV P9: Reduce irq_work vs guest decrementer races

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 06/03/2021 02:06, Nicholas Piggin wrote:
> irq_work's use of the DEC SPR is racy with guest<->host switch and guest
> entry which flips the DEC interrupt to guest, which could lose a host
> work interrupt.
> 
> This patch closes one race, and attempts to comment another class of
> races.
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   arch/powerpc/kvm/book3s_hv.c | 15 ++++++++++++++-
>   1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 6f3e3aed99aa..b7a88960ac49 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -3704,6 +3704,18 @@ 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);
>   
> +	/*
> +	 * When setting DEC, we must always deal with irq_work_raise via NMI vs
> +	 * setting DEC. The problem occurs right as we switch into guest mode
> +	 * if a NMI hits and sets pending work and sets DEC, then that will
> +	 * apply to the guest and not bring us back to the host.
> +	 *
> +	 * irq_work_raise could check a flag (or possibly LPCR[HDICE] for
> +	 * example) and set HDEC to 1? That wouldn't solve the nested hv
> +	 * case which needs to abort the hcall or zero the time limit.
> +	 *
> +	 * XXX: Another day's problem.
> +	 */
>   	mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
>   
>   	if (kvmhv_on_pseries()) {
> @@ -3838,7 +3850,8 @@ 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 - tb);
> +	set_dec_or_work(local_paca->kvm_hstate.dec_expires - tb);

set_dec_or_work() will write local_paca->kvm_hstate.dec_expires - tb - 1 
to SPRN_DEC which is not exactly the same, is this still alright?


> +
>   	mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
>   
>   	kvmhv_load_host_pmu();
> 

-- 
Alexey