Re: [Xen-devel] [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	stefan@agner.ch, Masahiro Yamada <yamada.masahiro@socionext.com>,
	Will Deacon <will@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sasha Levin <sashal@kernel.org>,
	sstabellini@kernel.org, James Morris <jmorris@namei.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	xen-devel@lists.xenproject.org,
	Vladimir Murzin <vladimir.murzin@arm.com>,
	Marc Zyngier <marc.zyngier@arm.com>,
	alexios.zavras@intel.com, boris.ostrovsky@oracle.com,
	allison@lohutok.net, jgross@suse.com, steve.capper@arm.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	LKML <linux-kernel@vger.kernel.org>,
	James Morse <james.morse@arm.com>,
	info@metux.net
Subject: Re: [Xen-devel] [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call
Date: Fri, 22 Nov 2019 00:53:12 +0000	[thread overview]
Message-ID: <20191122005311.GI25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <CA+CK2bAm0r8zLMz_gdq30zF8io5RzVnbXFSV9NkyT_uUxKJwLA@mail.gmail.com>

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.

-- 
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