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

[Russell King - ARM Linux admin <linux@xxxxxxxxxxxxxxx>]

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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