linux-kernel - Re: [PATCH v6 10/24] arm64: irqflags: Use ICC_PMR

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Date:   Mon, 10 Dec 2018 14:39:19 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Julien Thierry <julien.thierry@....com>
Cc:     daniel.thompson@...aro.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        marc.zyngier@....com, will.deacon@....com,
        christoffer.dall@....com, linux-kernel@...r.kernel.org,
        james.morse@....com, Oleg Nesterov <oleg@...hat.com>,
        joel@...lfernandes.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 10/24] arm64: irqflags: Use ICC_PMR_EL1 for interrupt
 masking

On Thu, Dec 06, 2018 at 09:50:18AM +0000, Julien Thierry wrote:
> On 05/12/18 18:26, Catalin Marinas wrote:
> > On Wed, Dec 05, 2018 at 04:55:54PM +0000, Julien Thierry wrote:
> >> On 04/12/18 17:36, Catalin Marinas wrote:
> >>> On Mon, Nov 12, 2018 at 11:57:01AM +0000, Julien Thierry wrote:
> >>>> diff --git a/arch/arm64/include/asm/irqflags.h b/arch/arm64/include/asm/irqflags.h
> >>>> index 24692ed..e0a32e4 100644
> >>>> --- a/arch/arm64/include/asm/irqflags.h
> >>>> +++ b/arch/arm64/include/asm/irqflags.h
> >>>> @@ -18,7 +18,27 @@
> >>>>  
> >>>>  #ifdef __KERNEL__
> >>>>  
> >>>> +#include <asm/alternative.h>
> >>>> +#include <asm/cpufeature.h>
> >>>>  #include <asm/ptrace.h>
> >>>> +#include <asm/sysreg.h>
> >>>> +
> >>>> +
> >>>> +/*
> >>>> + * When ICC_PMR_EL1 is used for interrupt masking, only the bit indicating
> >>>> + * whether the normal interrupts are masked is kept along with the daif
> >>>> + * flags.
> >>>> + */
> >>>> +#define ARCH_FLAG_PMR_EN 0x1
> >>>> +
> >>>> +#define MAKE_ARCH_FLAGS(daif, pmr)					\
> >>>> +	((daif) | (((pmr) >> GIC_PRIO_STATUS_SHIFT) & ARCH_FLAG_PMR_EN))
> >>>> +
> >>>> +#define ARCH_FLAGS_GET_PMR(flags)				 \
> >>>> +	((((flags) & ARCH_FLAG_PMR_EN) << GIC_PRIO_STATUS_SHIFT) \
> >>>> +		| GIC_PRIO_IRQOFF)
> >>>> +
> >>>> +#define ARCH_FLAGS_GET_DAIF(flags) ((flags) & ~ARCH_FLAG_PMR_EN)
> >>>
> >>> I wonder whether we could just use the PSR_I_BIT here to decide whether
> >>> to set the GIC_PRIO_IRQ{ON,OFF}. We could clear the PSR_I_BIT in
> >>> _restore_daif() with an alternative.
> >>
> >> So, the issue with it is that some contexts might be using PSR.I to
> >> disable interrupts (any contexts with async errors or debug exceptions
> >> disabled, kvm guest entry paths, pseudo-NMIs, ...).
> >>
> >> If any of these contexts calls local_irq_save()/local_irq_restore() or
> >> local_daif_save()/local_daif_restore(), by only relying on PSR_I_BIT to
> >> represent the PMR status, we might end up clearing PSR.I when we shouldn't.
> >>
> >> I'm not sure whether there are no callers of these functions in those
> >> context. But if that is the case, we could simplify things, yes.
> > 
> > There are callers of local_daif_save() (3) and local_daif_mask() (7) but
> > do they all need to disable the pseudo-NMIs?
> 
> Hmmm, I really think that both of those should be disabling NMIs.
> Otherwise, if we take an NMI, the first thing the el1_irq handler is
> going to do is "enable_da_f()" which could lead to potential issues.
> 
> One thing that could be done is:
> - local_daif_save() and local_daif_mask() both mask all daif bits
> (taking care to represent PMR value in the I bit of the saved flags)
> - local_daif_restore() restores da_f as expected and decides values to
> put for PMR and PSR.I as follows:
> 	* do the da_f restore
> 	* if PSR.A bit is cleared in the saved flags, then we also do a start_nmi()
> 
> However, this would not work with a local_daif_save()/restore() on the
> return path of an NMI because I think it is the only context with NMIs
> "stopped" that can take aborts. I can add a WARN_ON(in_nmi()) for
> local_daif_restore() if that doesn't affect performance too much.

FTR, as we discussed this in the office, the conclusion (IIUC) we got to
was: leave the *_daif_*() functions unchanged, touching all the
corresponding PSTATE bits, but change the arch_local_irq_*() macros to
only touch the PMR when the feature is enabled.

-- 
Catalin