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

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Date:   Fri, 22 Nov 2019 00:34:03 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, steve.capper@....com,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        James Morse <james.morse@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        allison@...utok.net, info@...ux.net, alexios.zavras@...el.com,
        sstabellini@...nel.org, boris.ostrovsky@...cle.com,
        jgross@...e.com, stefan@...er.ch,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 1/3] arm/arm64/xen: use C inlines for privcmd_call

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.

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