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Message-ID: <Yn0YdPNG/Q3lf+4G@hirez.programming.kicks-ass.net>
Date: Thu, 12 May 2022 16:23:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Alexander Potapenko <glider@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
"H . J . Lu" <hjl.tools@...il.com>,
Andi Kleen <ak@...ux.intel.com>,
Rick Edgecombe <rick.p.edgecombe@...el.com>,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 05/10] x86/mm: Provide untagged_addr() helper
On Thu, May 12, 2022 at 03:06:38PM +0200, Thomas Gleixner wrote:
> #define untagged_addr(addr) ({ \
> u64 __addr = (__force u64)(addr); \
> \
> __addr &= current->thread.lam_untag_mask; \
> (__force __typeof__(addr))__addr; \
> })
>
> No conditionals, fast _and_ correct. Setting this untag mask up once
> when LAM is enabled is not rocket science.
But that goes wrong if someone ever wants to untag a kernel address and
not use the result for access_ok().
I'd feel better about something like:
s64 __addr = (addr);
s64 __sign = __addr;
__sign >>= 63;
__sign &= lam_untag_mask;
__addr &= lam_untag_mask;
__addr |= __sign;
__addr;
Which simply extends bit 63 downwards -- although possibly there's an
easier way to do that, this is pretty gross.
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