linux-kernel - Re: [RFC, PATCHv1 00/28] 5-level paging

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Message-ID: <13962749.Q2mLWEctkQ@wuerfel>
Date:   Fri, 09 Dec 2016 11:24:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        maxim.kuvyrkov@...aro.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, broonie@...nel.org,
        schwidefsky@...ibm.com
Subject: Re: [RFC, PATCHv1 00/28] 5-level paging

On Friday, December 9, 2016 6:01:30 AM CET Ingo Molnar wrote:
> >   - Handle opt-in wider address space for userspace.
> > 
> >     Not all userspace is ready to handle addresses wider than current
> >     47-bits. At least some JIT compiler make use of upper bits to encode
> >     their info.
> > 
> >     We need to have an interface to opt-in wider addresses from userspace
> >     to avoid regressions.
> > 
> >     For now, I've included testing-only patch which bumps TASK_SIZE to
> >     56-bits. This can be handy for testing to see what breaks if we max-out
> >     size of virtual address space.
> 
> So this is just a detail - but it sounds a bit limiting to me to provide an 'opt 
> in' flag for something that will work just fine on the vast majority of 64-bit 
> software.
> 
> Please make this an opt out compatibility flag instead: similar to how we handle 
> address space layout limitations/quirks ABI details, such as ADDR_LIMIT_32BIT, 
> ADDR_LIMIT_3GB, ADDR_COMPAT_LAYOUT, READ_IMPLIES_EXEC, etc.

We've had a similar discussion about JIT software on ARM64, which has a wide
range of supported page table layouts and some software wants to limit that
to a specific number.

I don't remember the outcome of that discussion, but I'm adding a few people
to Cc that might remember.

There have also been some discussions in the past to make the depth of the
page table a per-task decision on s390, since you may have some tasks that
run just fine with two or three levels of paging while another task actually
wants the full 64-bit address space. I wonder how much extra work this would
be on top of the boot-time option.

	Arnd