linux-kernel - Re: [PATCH v6 08/24] arm64: Unmask PMR before going idle

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Message-ID: <20181130133719.zw7wcklqayiebgap@lakrids.cambridge.arm.com>
Date:   Fri, 30 Nov 2018 13:37:19 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        daniel.thompson@...aro.org, joel@...lfernandes.org,
        marc.zyngier@....com, christoffer.dall@....com,
        james.morse@....com, catalin.marinas@....com, will.deacon@....com
Subject: Re: [PATCH v6 08/24] arm64: Unmask PMR before going idle

On Fri, Nov 30, 2018 at 10:55:47AM +0000, Julien Thierry wrote:
> On 29/11/18 17:44, Mark Rutland wrote:
> > On Mon, Nov 12, 2018 at 11:56:59AM +0000, Julien Thierry wrote:

> >> +	mov	x2, #GIC_PRIO_IRQON
> >> +	msr_s	SYS_ICC_PMR_EL1, x2		// unmask PMR
> >>  	dsb	sy				// WFI may enter a low-power mode
> > 
> > Is the DSB SY sufficient and necessary to synchronise the update of
> > SYS_ICC_PMR_EL1? We don't need an ISB too?
> 
> DSB SY is necessary when we unmask interrupts to make sure that the
> redistributor sees the update to PMR before we do WFI. My understanding
> is that the resdistributor is free to stop forwarding interrupts to the
> CPU interface if from its point of view those interrupts don't have a
> high enough priority.
> 
> As for the ISB, I don't think we need one because writes to PMR are
> self-synchronizing, so the write to PMR should be seen before DSB SY and
> wfi.

Having looked at ARM IHI 0069D, 8.1.6 "Observability of the effects of
accesses to the GIC registers", I think I agree. 

My specific concern was that a CPU might complete the DSB before the
MSR, but I think it's clear per the GIC spec it's clear that an ISB is
not expected between the MSR and DSB, even if that's unusual.

> >>  	wfi
> >> +	msr_s	SYS_ICC_PMR_EL1, x1		// restore PMR
> > 
> > Likewise, we don't need any barriers here before we poke DAIF?
> 
> Here we don't need DSB SY because the value being restored is either:
> - GIC_PRIO_IRQON which is the same as the current value, the
> redistributor is already aware of it.
> - GIC_PRIO_IRQOFF and the self-synchronization of PMR ensures that no
> interrupts with priorities lower than the value of PMR can be taken
> (this does not require to be seen by the redistributor).
> 
> For the ISB, I have this small doubt about whether it is needed between
> WFI and MSR PMR. But there is this bit in the ARM ARM section D12.1.3
> "General behavior of accesses to the AArch64 System registers",
> subsection "Synchronization requirements for AArch64 System registers":
> 
> "Direct writes using the instructions in Table D11-2 on page D11-2660
> require synchronization before software can rely on the effects of
> changes to the System registers to affect instructions appearing in
> program order after the direct write to the System register. Direct
> writes to these registers are not allowed to affect any instructions
> appearing in program order before the direct write."
> 
> ICC_PMR_EL1 is part of the mentioned table.

I think that's a defect in the ARM ARM, given it disagrees with the GIC
spec.

> And reordering the direct write to PMR before the WFI would definitely
> affect the WFI instruction, so my interpretation is that this would
> not be allowed by the architecture. So I don't think we need the ISB
> either, but my understanding could be wrong.

We already assume that a DSB can't be re-ordered w.r.t. the WFI, so as
long as the DSB can't complete before the MSR, I think we're good.

> > 
> >> +	msr	daif, x0			// restore I bit
> >>  	ret
> >>  ENDPROC(cpu_do_idle)
> > 
> > If we build without CONFIG_ARM64_PSEUDO_NMI surely we don't want to emit
> > the alternative?
> > 
> > How about we move this to C, and have something like the below?
> > 
> > For the !CONFIG_ARM64_PSEUDO_NMI case it generates identical assembly to the
> > existing cpu_do_idle(). Note that I've assumed we don't need barriers, which
> > (as above) I'm not certain of.
> > 
> > Thanks,
> > Mark.
> > 
> > ---->8----
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 7f1628effe6d..ccd2ad8c5e2f 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -73,6 +73,40 @@ EXPORT_SYMBOL_GPL(pm_power_off);
> >  
> >  void (*arm_pm_restart)(enum reboot_mode reboot_mode, const char *cmd);
> >  
> > +static inline void __cpu_do_idle(void)
> > +{
> > +       /* WFI may enter a low-power mode */
> > +       dsb(sy);
> > +       wfi();
> > +}
> > +
> > +/*
> > + * When using priority masking we need to take extra care, etc.
> > + */
> > +static inline void __cpu_do_idle_irqprio(void)
> > +{
> > +       unsigned long flags = arch_local_irq_save();
> 
> The issue with this is that in patch 10, arch_local_irq_* functions
> toggle PMR rather than PSR.I.
> 
> I could use local_daif_mask but I don't think disabling debug and async
> is good. Otherwise and can do a small bit of  inline assembly and have
> something like:

Can we factor out the existing arch_local_irq_save() somehow?

Thanks,
Mark.