Date: Wed, 1 Apr 2015 16:15:41 +0100
From: Dave Martin <Dave.Martin@arm.com>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: linaro-kernel@lists.linaro.org, patches@linaro.org,
        Marc Zyngier <marc.zyngier@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org,
        John Stultz <john.stultz@linaro.org>,
        Andrew Thoelke <andrew.thoelke@arm.com>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH 0/7] Pseudo-NMI for arm64 using ICC_PMR_EL1 (GICv3)
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Apologies for the slow reply... :/

Anyway,

On Mon, Mar 23, 2015 at 06:47:53PM +0000, Daniel Thompson wrote:
> On 20/03/15 15:45, Dave Martin wrote:
> >On Wed, Mar 18, 2015 at 02:20:21PM +0000, Daniel Thompson wrote:
> >>This patchset provides a pseudo-NMI for arm64 kernels by reimplementing
> >>the irqflags macros to modify the GIC PMR (the priority mask register is
> >>accessible as a system register on GICv3 and later) rather than the
> >>PSR. The pseudo-NMI changes are support by a prototype implementation of
> >>arch_trigger_all_cpu_backtrace that allows the new code to be exercised.

Minor nit: the "pseudo NMI" terminology could lead to confusion if
something more closely resembling a real NMI comes along.

I'll have to have a think, but nothing comes to mind right now...

[...]

> >>  3. Requires GICv3+ hardware together with firmware support to enable
> >>     GICv3 features at EL3. If CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS is
> >>     enabled the kernel will not boot on older hardware. It will be hard
> >>     to diagnose because we will crash very early in the boot (i.e.
> >>     before the call to start_kernel). Auto-detection might be possible
> >>     but the performance and code size cost of adding conditional code to
> >>     the irqflags macros probably makes it impractical. As such it may
> >>     never be possible to remove this limitation (although it might be
> >>     possible to find a way to survive long enough to panic and show the
> >>     results on the console).
> >
> >This can (and should) be done via patching -- otherwise we risk breaking
> >single kernel image for GICv2+v3.
> 
> Do you mean real patching (hunting down all those inlines and
> rewrite them) or simply implementing irqflags with an ops table? If
> the former I didn't look at this because I didn't release we could
> do that...

A generic patching framework was introduced by Andre Przywara in this
patch:

e039ee4 arm64: add alternative runtime patching

I believe you should be able to use this to patch between DAIF and
ICC_PMR accesses.

You should be able to find examples of this framework being used by
grepping.  I've not played with it myself yet.

[...]

> >>  5. There is no code in el1_irq to detect NMI and switch from IRQ to NMI
> >>     handling. This means all the irq handling machinary is re-entered in
> >>     order to handle the NMI. This not safe and deadlocks are likely.
> >>     This is a severe limitation although, in this proof-of-concept
> >>     work, NMI can only be triggered by SysRq-L or severe kernel damage.
> >>     This means we just about get away with it for simple test (lockdep
> >>     detects that we are doing wrong and shows a backtrace). This is
> >>     definitely the first thing that needs to be tackled to take this
> >>     code further.
> >
> >Indeed, and this does look a bit weird at present... it took me a
> >while to figure out where NMIs could possibly be coming from in
> >this series.
> 
> My plan was to check the running priority register early in el1_irq
> and branch to a handler specific to NMI when the priority indicates
> we are handling a pseudo-NMI.

Sounds reasonable.

> >>Note also that alternative approaches to implementing a pseudo-NMI on
> >>arm64 are possible but only through runtime cooperation with other
> >>software components in the system, potentially both those running at EL3
> >>and at secure EL1. I should like to explore these options in future but,
> >
> >For the KVM case, vFIQ is an obvious choice, but you're correct that
> >all other scenarios would require cooperation from a separate hypervisor/
> >firmware etc.
> >
> >Ideally, we should avoid having multiple ways of implementing the same
> >thing.
> >
> >>as far as I know, this is the only sane way to provide NMI-like features
> >>whilst being implementable entirely in non-secure EL1[1]
> >>
> >>[1] Except for a single register write to ICC_SRE_EL3 by the EL3
> >>     firmware (and already implemented by ARM trusted firmware).
> >
> >Even that would require more of the memory-mapped GIC CPU interface
> >to be NS-accessible than is likely to be the case on product
> >platforms.  Note also that the memory-mapped interface is not
> >mandated for GICv3, so some platforms may simply not have it.
> 
> Perhaps I used clumsy phrasing here.
> 
> There is a main difference I care about is between runtime
> cooperation and boot-time cooperation. The approach I have taken in
> the patchset requires boot time cooperation (to configure GIC
> appropriately) but no runtime cooperation.

I think that's reasonable.  Any new boot requirements will need to be
documented (probably in booting.txt) as part of the final series
and alongside the relevant Kconfig option.

> >Some other generalities that don't seem to be addressed yet:
> >
> >	* How are NMIs prioritised with respect to other interrupts and
> >	  exceptions?  This needs to be concretely specified.  A sensible
> >	  answer would probably be that the effect is to split the
> >	  existing single-priority IRQ into two bands: ordinary IRQs
> >	  and NMIs.  Prioritisation against FIQ and other exceptions
> >	  would be unaffected.
> >
> >	  I think this is effectively what you've implemented so far.
> 
> Pretty much. Normal interrupts run at the default priority and NMIs
> run at default priority but with bit 6 cleared.
> 
> In addition I would expect most kernel exception handlers to unmask
> the I-bit as soon as the context has been saved. This allows them to
> be pre-empted by an NMI.

Yep, that matches my expectation.

> 
> >
> >	* Should it be possible to map SPIs as NMIs?  How would they
> >	  be configured/registed?  Should it be possible to register
> >	  multiple interrupts as NMIs?
> 
> Yes, although not quite yet.
> 
> The work on arm64 is following in the footsteps of similar work for arm.
> 
> My initial ideas are here (although as you can see from the review
> I've got a *long* way to go):
>   http://thread.gmane.org/gmane.linux.kernel/1871163
> 
> However the basic theme is:
> 
> 1. Use existing interrupt API to register NMI handlers (with special
>    flag).
> 
> 2. Flag makes callback to irqchip driver. In case of GICv3 this would
>    alter the priority of interrupt (for ARMv7+FIQ it would also change
>    interrupt group but this is not needed on ARMv8+GICv3).
> 
> 3. "Simple" demux. We cannot traverse all the IRQ data structures from
>    NMI due to locks within the structures so we need some simplifying
>    assumptions. My initial simplifying assumptions were:
> 
>    a) NMI only for first level interrupt controller (i.e. peripherals
>       directly attached to this controller).
> 
>    b) No shared interrupt lines.

Do other arches have ways of addressing the same problems?

> Based on tglx's review I'm working on the basis that b) above is
> simplication too many but I've not yet had the chance to go back and
> have anyother go.

I think that it's best to avoid adding arbitrary restrictions that
make this look excessively different from working with a regular
irqchip, unless there is really some fundamental constraint in play.

> As you can see from the reviews I have a bit of work to do in orde
> 
> >	* What about interrupt affinity?
> 
> It should "just work" as normal if I can get the rest of the
> interrupt system right. Do you foresee a specific problem?

So long as NMI-ness is just an extra flag when registering an
interrupt, things should probably work.  I was wondering about
special cases like perf (PPI on sensible SoCs) versus, say,
debug UART (SPI).

> >Some other points:
> >
> >	* I feel uneasy about using reserved SPSR fields to store
> >	  information.  This is probably OK for now, but it might
> >	  be cleaner simply to save/restore the PMR directly.
> >
> >	  Providing that the affected bit is cleared before writing
> >	  to the SPSR (as you do already in kernel_exit) looks
> >	  workable, but wonder whether the choice of bit should be
> >	  UAPI -- it may have to change in the future.
> 
> I agree we ought to keep this out of the uapi.
> 
> Regarding stealing a bit from the SPSR this was mostly an
> implementation convenience. It might be interesting (eventually) to
> benchmark it against a more obvious approach.

I think your current approach is OK for now, at least while the
series is under development.

> >	* You can probably thin out the ISBs.
> >
> >	  I believe that the via the system register interface,
> >	  the GICC PMR is intended to be self-synchronising.
> 
> That sounds great. I've just found the relevant line in the ARMv8
> manual... I'd overlooked that before.
> 
> >	* The value BPR resets to is implementation-dependent.
> >	  It should be initialised on each CPU if we are going to rely
> >	  on its value, on all platforms.  This isn't specific to FVP.
> 
> Really? As mentioned I only have a GICv2 spec but on that revision
> the reset value is the minimum supported value (i.e. the same effect
> as attempting to set it to zero). In other words it is technically
> implementation-dependent but nevertheless defaults to a setting that
> avoids any weird "globbing" effect on the interrupt priorities.
> 
> On FVP something has reached in and changed the BPR for CPU0 from
> its proper reset value (all oter CPUs have correct reset value). Of
> course that could be the firmware rather than the FVP itself that
> has caused this.

Quite possibly.  Of course, there is a strong possibility that some
real firmware will also do this (and never get fixed).

Forcing BPR to a sane state from Linux makes sense, since we can
do it.

> I guess it is good practice for a driver to re-establish the reset
> value for register it owns and cares about but nevertheles I still
> expect this register to as-reset when we handover to the kernel.
> 
> >	* Is ICC_CTLR_EL1.CBPR set correctly?
> 
> I've never checked. On GICv2 that would be secure state so to be
> honest I didn't think its value is any of my business.

If we have a dependency on how this is set up, it needs to be 
documented alongside the other booting requirements.

Cheers
---Dave

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