linux-kernel - Re: [PATCH v4] arm64: Add support for new control bits CTR_EL0.DIC and CTR

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Date:   Thu, 22 Feb 2018 16:28:03 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Mark Rutland <mark.rutland@....com>,
        Shanker Donthineni <shankerd@...eaurora.org>
Cc:     Philip Elcan <pelcan@...eaurora.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4] arm64: Add support for new control bits CTR_EL0.DIC
 and CTR_EL0.IDC

[Apologies to keep elbowing in, and if I'm being thick here...]

On 22/02/18 15:22, Mark Rutland wrote:
> On Thu, Feb 22, 2018 at 08:51:30AM -0600, Shanker Donthineni wrote:
>> +#define CTR_B31_SHIFT		31
> 
> Since this is just a RES1 bit, I think we don't need a mnemonic for it,
> but I'll defer to Will and Catalin on that.
> 
>>   ENTRY(invalidate_icache_range)
>> +#ifdef CONFIG_ARM64_SKIP_CACHE_POU
>> +alternative_if ARM64_HAS_CACHE_DIC
>> +	mov	x0, xzr
>> +	dsb	ishst
>> +	isb
>> +	ret
>> +alternative_else_nop_endif
>> +#endif
> 
> As commented on v3, I don't believe you need the DSB here. If prior
> stores haven't been completed at this point, the existing implementation
> would not work correctly here.

True in terms of ordering between stores prior to entry and the IC IVAU 
itself, but what about the DSH ISH currently issued *after* the IC IVAU 
before returning? Is provably impossible that existing callers might be 
relying on that ordering *anything*, or would we risk losing something 
subtle by effectively removing it?

Robin.