linux-kernel - Re: [RFC PATCH 3/9] irqchip: GIC: Convert to EOImode == 1

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Date:	Wed, 25 Jun 2014 08:18:46 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	"eric.auger@...aro.org" <eric.auger@...aro.org>
Subject: Re: [RFC PATCH 3/9] irqchip: GIC: Convert to EOImode == 1

On Wed, Jun 25, 2014 at 8:03 AM, Marc Zyngier <marc.zyngier@....com> wrote:
> On Wed, Jun 25 2014 at 01:50:12 PM, Rob Herring <robherring2@...il.com> wrote:
>> On Wed, Jun 25, 2014 at 4:28 AM, Marc Zyngier <marc.zyngier@....com> wrote:
>>> So far, GICv2 has been used in with EOImode == 0. The effect of this
>>> mode is to perform the priority drop and the deactivation of the
>>> interrupt at the same time.
>>>
>>> While this works perfectly for Linux (we only have a single priority),
>>> it causes issues when an interrupt is forwarded to a guest, and when
>>> we want the guest to perform the EOI itself.
>>>
>>> For this case, the GIC architecture provides EOImode == 1, where:
>>> - A write to the EOI register drops the priority of the interrupt and leaves
>>> it active. Other interrupts at the same priority level can now be taken,
>>> but the active interrupt cannot be taken again
>>> - A write to the DIR marks the interrupt as inactive, meaning it can
>>> now be taken again.
>>>
>>> We only enable this feature when booted in HYP mode. Also, as most device
>>> trees are broken (they report the CPU interface size to be 4kB, while
>>> the GICv2 CPU interface size is 8kB), output a warning if we're booted
>>> in HYP mode, and disable the feature.
>>
>> Why not fix-up the size so the feature can be enabled?
>
> Is it a bet we're willing to take? We'd end-up with a kernel that
> doesn't boot if the DT was actually right. If we stay with EOImode==0,
> we can still boot (KVM will probably be broken though).

I think so. Seems like your last statement answers this. Why is KVM
not working on my system seems like a much more likely and frequent
support issue than a potentially broken system.

The only place I really could see it be broken is an SBSA system doing
the address swizzling trick with the gic-400 to get 64KB spaced
regions but does not use the 60KB aligned cpu interface address. But
DTBs are hardly stable for 64-bit systems and can be updated.

Rob
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