linux-kernel - Re: [RFC PATCH 01/10] arm64: feature registers: Documentation

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Message-ID: <20150910155523.GH6281@e103592.cambridge.arm.com>
Date:	Thu, 10 Sep 2015 16:55:25 +0100
From:	Dave Martin <Dave.Martin@....com>
To:	"Suzuki K. Poulose" <Suzuki.Poulose@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Mark Rutland <Mark.Rutland@....com>,
	"aph@...hat.com" <aph@...hat.com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"edward.nevill@...aro.org" <edward.nevill@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC PATCH 01/10] arm64: feature registers: Documentation

On Tue, Aug 11, 2015 at 04:37:46PM +0100, Suzuki K. Poulose wrote:
> On 11/08/15 15:23, Catalin Marinas wrote:
> >On Mon, Aug 10, 2015 at 07:48:48PM +0200, Ard Biesheuvel wrote:
> >>>On 10/08/15 17:06, Catalin Marinas wrote:
> >>>>And to debunk some of the counter arguments:
> >>>>
> >>>>a) Running out of HWCAP bits - I really doubt this, we can always
> >>>>     introduce 64 more via a new elf_hwcapX
> >>
> >>Note that ELF_HWCAP is also wired into ifunc resolution of GNU
> >>indirect functions, which looks like a useful feature although it
> >>isn't used that widely yet.
> >
> >I forgot to mention, we also need an HWCAP_CPUID with these patches when
> >we expose the MRS interface. The ifunc resolver could use MRS when
> >available. But I would still keep adding HWCAP bits for new features,
> >even if we risk running out of the 64-bit we have now.
> >
> 
> Sure, I will add the HWCAP_CPUID in the next version of the series.

+1

Playing with this, I realise that I get a splat if my userspace code
tries to do an MRS for an ID register when this series is absent -- we
need an hwcap that we can check first.

Cheers
---Dave

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