linux-kernel - Re: [PATCH v2 5/5] uaccess: Rename user_access_begin/end() to user_full_access

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Date:   Tue, 21 Apr 2020 10:12:56 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Al Viro <viro@...iv.linux.org.uk>, Will Deacon <will@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>, Peter Anvin <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        intel-gfx@...ts.freedesktop.org
Subject: Re: [PATCH v2 5/5] uaccess: Rename user_access_begin/end() to
 user_full_access_begin/end()

On Tue, Apr 21, 2020 at 03:49:19AM +0100, Al Viro wrote:
> 	The only source I'd been able to find speeks of >= 60 cycles
> (and possibly much more) for non-pipelined coprocessor instructions;
> the list of such does contain loads and stores to a bunch of registers.
> However, the register in question (p15/c3) has only store mentioned there,
> so loads might be cheap; no obvious reasons for those to be slow.
> That's a question to arm folks, I'm afraid...  rmk?

I have no information on that; instruction timings are not defined
at architecture level (architecture reference manual), nor do I find
information in the CPU technical reference manual (which would be
specific to the CPU). Instruction timings tend to be implementation
dependent.

I've always consulted Will Deacon when I've needed to know whether
an instruction is expensive or not.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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