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>,
	boris.ostrovsky@oracle.com, 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, Thomas Gleixner <tglx@linutronix.de>,
	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 00:34:03 +0000	[thread overview]
Message-ID: <20191122003403.GG25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <CA+CK2bDtADA2eVwJAUEPhpic8vXWegh8yLjo6Q6WmXZDxAfJpA@mail.gmail.com>

On Thu, Nov 21, 2019 at 07:30:41PM -0500, Pavel Tatashin wrote:
> > > +#ifdef CONFIG_CPU_SW_DOMAIN_PAN
> > > +static __always_inline void uaccess_enable(void)
> > > +{
> > > +     unsigned long val = DACR_UACCESS_ENABLE;
> > > +
> > > +     asm volatile("mcr p15, 0, %0, c3, c0, 0" : : "r" (val));
> > > +     isb();
> > > +}
> > > +
> > > +static __always_inline void uaccess_disable(void)
> > > +{
> > > +     unsigned long val = DACR_UACCESS_ENABLE;
> 
> Oops, should be DACR_UACCESS_DISABLE.
> 
> > > +
> > > +     asm volatile("mcr p15, 0, %0, c3, c0, 0" : : "r" (val));
> > > +     isb();
> > > +}
> >
> > Rather than inventing these, why not use uaccess_save_and_enable()..
> > uaccess_restore() around the Xen call?
> 
> Thank you for suggestion: uaccess_enable() and uaccess_disable() are
> common calls with arm64, so I will need them, but I think I can use
> set_domain() with DACR_UACCESS_DISABLE /DACR_UACCESS_ENABLE inside
> these inlines.

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.

-- 
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