Re: [PATCH] arm64: Relax ICC_PMR_EL1 accesses when ICC_CTLR_EL1.PMHE is clear

From: Marc Zyngier <marc.zyngier@arm.com>
To: Julien Thierry <julien.thierry.kdev@gmail.com>
Cc: Marc Zyngier <maz@kernel.org>, Will Deacon <will@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	James Morse <james.morse@arm.com>,
	huawei.libin@huawei.com, guohanjun@huawei.com,
	liwei391@huawei.com, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] arm64: Relax ICC_PMR_EL1 accesses when ICC_CTLR_EL1.PMHE is clear
Date: Thu, 1 Aug 2019 12:07:58 +0100	[thread overview]
Message-ID: <166cd7e1-4a4c-021b-eca6-f3f11fc4993d@arm.com> (raw)
In-Reply-To: <CAA3o8kyYnek=7boO_szVoNvQks8DFGN5s37ROQKqwyWZQZiXZw@mail.gmail.com>

On 01/08/2019 11:56, Julien Thierry wrote:
> On Thu, 1 Aug 2019 at 11:51, Marc Zyngier <marc.zyngier@arm.com> wrote:
>>
>> On 01/08/2019 11:41, Will Deacon wrote:
>>> On Tue, Jul 30, 2019 at 03:20:45PM +0100, Julien Thierry wrote:
>>>> From: Marc Zyngier <marc.zyngier@arm.com>
>>>>
>>>> The GICv3 architecture specification is incredibly misleading when it
>>>> comes to PMR and the requirement for a DSB. It turns out that this DSB
>>>> is only required if the CPU interface sends an Upstream Control
>>>> message to the redistributor in order to update the RD's view of PMR.
>>>>
>>>> This message is only sent when ICC_CTLR_EL1.PMHE is set, which isn't
>>>> the case in Linux. It can still be set from EL3, so some special care
>>>> is required. But the upshot is that in the (hopefuly large) majority
>>>> of the cases, we can drop the DSB altogether.
>>>>
>>>> This requires yet another capability and some more runtime patching.
>>>
>>> Hmm, does this actually require explicit runtime patching, or can we make
>>> things a bit simpler with a static key?
>>
>> The hunk in entry.S is the blocker, AFAICS. Do we have a way to express
>> static keys in asm?
>>
> 
> Not that I'm aware of. I could leave the alternative in entry.S and
> use a static_key for the pmr_sync() macro.

I'm not sure that helps. It means we end-up with two mechanisms to keep
in sync instead of a single one.

> Does it change much over all? I don't see the static key simplifying
> things too much, but I don't mind using that instead.

The complexity is the same. The added benefit is that we can control it
from the GIC code rather than the architecture code. But that's assuming
we can do it all using a static key...

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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