linux-kernel - Re: [PATCH 2/2] irqchip/gic: Identify and report any reserved SGI IDs

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Date:	Thu, 17 Dec 2015 19:26:10 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	patches@...aro.org, linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH 2/2] irqchip/gic: Identify and report any reserved SGI IDs

On Wed, Dec 16, 2015 at 05:47:09PM +0000, Marc Zyngier wrote:
> Hi Daniel,

Hi Marc

Thanks for the review.


> On 16/12/15 17:08, Daniel Thompson wrote:
> > It is possible for the secure world to reserve certain SGI IDs for itself.
> > Currently we have limited visibility of which IDs are safe to use for IPIs.
> > 
> > Modify the GIC initialization code to actively search for reserved SGI IDs
> > and report if any are found. Warn even more loudly if the reserved SGIs
> > overlap with the normal IPI range.
> > 
> > When run on an Inforce IFC6410 (Snapdragon 600) this code produces the
> > following messages:
> > ~~~ cut here ~~~
> > CPU0: Detected reserved SGI IDs: 14-15
> > CPU1: Detected reserved SGI IDs: 15
> > CPU2: Detected reserved SGI IDs: 15
> > CPU3: Detected reserved SGI IDs: 15
> > ~~~ cut here ~~~
> > 
> > Signed-off-by: Daniel Thompson <daniel.thompson@...aro.org>

BTW you *didn't* say "this code is pointless and I hate it"...

Does that mean I should be looking at adding similar code for GICv3+? I 
wanted to guage reactions to this sort of diagnostics before getting
carried away!


> > +
> > +		/*
> > +		 * Fiddle with the SGI set/clear registers to try identify
> > +		 * any IPIs that are reserved for secure world.
> > +		 */
> > +		bitmap_fill(sgi_mask, 16);
> > +
> > +		for (i = 0; i < 16; i++) {
> > +			void __iomem *set_reg =
> > +			    dist_base + GIC_DIST_SGI_PENDING_SET + (i & ~3);
> > +			void __iomem *clear_reg =
> > +			    dist_base + GIC_DIST_SGI_PENDING_CLEAR + (i & ~3);
> > +			unsigned long mask = cpu_mask << (8*(i%4));
> > +			unsigned long flags, pending, after_clear, after_set;
> 
> Please make these u32, as unsigned long is 64bit on arm64. Another thing
> to note is that GICD_CPEND{S,C}SGIRn are byte accessible, so you can
> save yourself some this hassle shifting things around and just write a
> single byte. You're already writing 16 times anyway...

Will do both.


> Another thing to consider is that these locations are only defined on
> GICv2 and not GICv1, so this patch is likely to cause trouble on older HW.

As presented the code relies on the RAZ/WI property of reserved
registers to avoid issues on GICv1; it does not report anything if there
appear to be know working SGIs on the assumption we are actually running
on a GICv1.

You'd prefer an explicit version check?


> > +
> > +			local_irq_save(flags);
> 
> Why do you need to do this? The CPU interface is not enabled yet, so I
> can't see how you could get an interrupt on this CPU.

Agreed. Can get rid of these.


> > +
> > +			/* record original value */
> > +			pending = readl_relaxed(set_reg);
> > +
> > +			/* clear, test, set, and test again */
> > +			writel_relaxed(mask, clear_reg);
> > +			after_clear = readl_relaxed(set_reg);
> > +			writel_relaxed(mask, set_reg);
> > +			after_set = readl_relaxed(set_reg);
> 
> It should be enough to write to the SET register, and read back, as the
> bit is RAZ/WI when the interrupt is Group-0.

Good point. Will simplify.


Daniel.
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