Thanks to address translation being performed out of order with respect to
loads and stores, it is possible for a CPU to take a translation fault when
accessing a page that was mapped by a different CPU.

For example, in the case that one CPU maps a page and then sets a flag to
tell another CPU:

	CPU 0
	-----

	MOV	X0, <valid pte>
	STR	X0, [Xptep]	// Store new PTE to page table
	DSB	ISHST
	ISB
	MOV	X1, #1
	STR	X1, [Xflag]	// Set the flag

	CPU 1
	-----

loop:	LDAR	X0, [Xflag]	// Poll flag with Acquire semantics
	CBZ	X0, loop
	LDR	X1, [X2]	// Translates using the new PTE

then the final load on CPU 1 can raise a translation fault because the
translation can be performed speculatively before the read of the flag and
marked as "faulting" by the CPU. This isn't quite as bad as it sounds
since, in reality, code such as:

	CPU 0				CPU 1
	-----				-----
	spin_lock(&lock);		spin_lock(&lock);
	*ptr = vmalloc(size);		if (*ptr)
	spin_unlock(&lock);			foo = **ptr;
					spin_unlock(&lock);

will not trigger the fault because there is an address dependency on CPU 1
which prevents the speculative translation. However, more exotic code where
the virtual address is known ahead of time, such as:

	CPU 0				CPU 1
	-----				-----
	spin_lock(&lock);		spin_lock(&lock);
	set_fixmap(0, paddr, prot);	if (mapped)
	mapped = true;				foo = *fix_to_virt(0);
	spin_unlock(&lock);		spin_unlock(&lock);

could fault. This can be avoided by any of:

	* Introducing broadcast TLB maintenance on the map path
	* Adding a DSB;ISB sequence after checking a flag which indicates
	  that a virtual address is now mapped
	* Handling the spurious fault

Given that we have never observed a problem due to this under Linux and
future revisions of the architecture are being tightened so that
translation table walks are effectively ordered in the same way as explicit
memory accesses, we no longer treat spurious kernel faults as fatal if an
AT instruction indicates that the access does not trigger a translation
fault.
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>