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Date: Tue, 23 Mar 2021 21:15:36 +1000
From: Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH v4 28/46] KVM: PPC: Book3S HV P9: Reduce irq_work vs guest
 decrementer races
To: Alexey Kardashevskiy <aik@ozlabs.ru>, kvm-ppc@vger.kernel.org
References: <20210323010305.1045293-1-npiggin@gmail.com>
 <20210323010305.1045293-29-npiggin@gmail.com>
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Excerpts from Nicholas Piggin's message of March 23, 2021 8:36 pm:
> Excerpts from Alexey Kardashevskiy's message of March 23, 2021 8:13 pm:
>>=20
>>=20
>> On 23/03/2021 12:02, Nicholas Piggin wrote:
>>> irq_work's use of the DEC SPR is racy with guest<->host switch and gues=
t
>>> entry which flips the DEC interrupt to guest, which could lose a host
>>> work interrupt.
>>>=20
>>> This patch closes one race, and attempts to comment another class of
>>> races.
>>>=20
>>> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
>>> ---
>>>   arch/powerpc/kvm/book3s_hv.c | 15 ++++++++++++++-
>>>   1 file changed, 14 insertions(+), 1 deletion(-)
>>>=20
>>> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.=
c
>>> index 1f38a0abc611..989a1ff5ad11 100644
>>> --- a/arch/powerpc/kvm/book3s_hv.c
>>> +++ b/arch/powerpc/kvm/book3s_hv.c
>>> @@ -3745,6 +3745,18 @@ 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);
>>>  =20
>>> +	/*
>>> +	 * 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);
>>>  =20
>>>   	if (kvmhv_on_pseries()) {
>>> @@ -3879,7 +3891,8 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *=
vcpu, u64 time_limit,
>>>   	vc->entry_exit_map =3D 0x101;
>>>   	vc->in_guest =3D 0;
>>>  =20
>>> -	mtspr(SPRN_DEC, local_paca->kvm_hstate.dec_expires - tb);
>>> +	set_dec_or_work(local_paca->kvm_hstate.dec_expires - tb);
>>=20
>>=20
>> set_dec_or_work() will write local_paca->kvm_hstate.dec_expires - tb - 1=
=20
>> to SPRN_DEC which is not exactly the same, is this still alright?
>>=20
>> I asked in v3 but it is probably lost :)
>=20
> Oh I did see that then forgot.
>=20
> It will write dec_expires - tb, then it will write 1 if it found irq_work
> was pending.

Ah you were actually asking about set_dec writing val - 1. I totally=20
missed that.

Yes that was an unintentional change. This is the way timer.c code works=20
with respect to the decrementers_next_tb value, so it's probably better=20
to make them so it seems like it should be okay (and better to bring the=20
KVM code up to match timer code rather than be different or the other
way around). The difference should be noted in the changelog though.

Thanks,
Nick