linux-kernel - Re: [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd

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Date:   Fri, 22 Nov 2019 00:53:12 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>, stefan@...er.ch,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Will Deacon <will@...nel.org>, boris.ostrovsky@...cle.com,
        Sasha Levin <sashal@...nel.org>, sstabellini@...nel.org,
        James Morris <jmorris@...ei.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        xen-devel@...ts.xenproject.org,
        Vladimir Murzin <vladimir.murzin@....com>,
        Marc Zyngier <marc.zyngier@....com>, alexios.zavras@...el.com,
        Thomas Gleixner <tglx@...utronix.de>, allison@...utok.net,
        jgross@...e.com, steve.capper@....com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>, info@...ux.net
Subject: Re: [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call

On Thu, Nov 21, 2019 at 07:39:22PM -0500, Pavel Tatashin wrote:
> > > That may be, but be very careful that you only use them in ARMv7-only
> > > code.  Using them elsewhere is unsafe as the domain register is used
> > > for other purposes, and merely blatting over it (as your
> > > uaccess_enable and uaccess_disable functions do) is unsafe.
> >
> > In fact, I'll turn that into a bit more than a suggestion.  I'll make
> > it a NAK on adding them to 32-bit ARM.
> >
> 
> That's fine, and I also did not want to change ARM 32-bit. But, do you
> have a suggestion how differentiate between arm64 and arm in
> include/xen/arm/hypercall.h without ugly ifdefs?

Sorry, I don't.

I'm surprised ARM64 doesn't have anything like that, but I suspect
that's because they don't need to do a save/restore type operation.
Whereas, 32-bit ARM does very much need the save/restore behaviour
(although not in this path.)

The problem is, turning uaccess_enable/disable into C code means
that it's open to being used elsewhere in the kernel (ooh, a couple
of useful looking functions that work on both architectures!  I can
use that too!) and then we end up with stuff breaking subtly.  It's
the potential for subtle breakage that is making me NAK the idea of
adding the inline C functions.

Given the two have diverged, the only answer is ifdefs, sorry.

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