linux-kernel - Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control the tagged user addresses ABI

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Message-ID: <20190802105038.GC4175@arrakis.emea.arm.com>
Date:   Fri, 2 Aug 2019 11:50:38 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Kevin Brodsky <kevin.brodsky@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, kvm@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        Yishai Hadas <yishaih@...lanox.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Alexander Deucher <Alexander.Deucher@....com>,
        Christian Koenig <Christian.Koenig@....com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Jens Wiklander <jens.wiklander@...aro.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Leon Romanovsky <leon@...nel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Dave Martin <Dave.Martin@....com>,
        Khalid Aziz <khalid.aziz@...cle.com>, enh <enh@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christoph Hellwig <hch@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Robin Murphy <robin.murphy@....com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v19 02/15] arm64: Introduce prctl() options to control
 the tagged user addresses ABI

On Thu, Aug 01, 2019 at 09:45:05AM -0700, Dave Hansen wrote:
> On 8/1/19 5:38 AM, Kevin Brodsky wrote:
> > This patch series only changes what is allowed or not at the syscall
> > interface. It does not change the address space size. On arm64, TBI (Top
> > Byte Ignore) has always been enabled for userspace, so it has never been
> > possible to use the upper 8 bits of user pointers for addressing.
> 
> Oh, so does the address space that's available already chop that out?

Yes. Currently the hardware only supports 52-bit virtual addresses. It
could be expanded (though it needs a 5th page table level) to 56-bit VA
but it's not currently on our (hardware) plans. Beyond 56-bit, it cannot
be done without breaking the software expectations (and hopefully I'll
retire before we need this ;)).

> > If other architectures were to support a similar functionality, then I
> > agree that a common and more generic interface (if needed) would be
> > helpful, but as it stands this is an arm64-specific prctl, and on arm64
> > the address tag is defined by the architecture as bits [63:56].
> 
> It should then be an arch_prctl(), no?

I guess you just want renaming SET_TAGGED_ADDR_CTRL() to
arch_prctl_tagged_addr_ctrl_set()? (similarly for 'get')

-- 
Catalin