Subject: Re: [PATCH v5 02/19] ARM: LPAE: add ISBs around MMU enabling code
From: Catalin Marinas <catalin.marinas@arm.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Will Deacon <Will.Deacon@arm.com>
In-Reply-To: <20110509120509.GR27807@n2100.arm.linux.org.uk>
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Organization: ARM Limited
Date: Mon, 09 May 2011 16:01:56 +0100
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On Mon, 2011-05-09 at 13:05 +0100, Russell King - ARM Linux wrote:
> On Mon, May 09, 2011 at 11:59:54AM +0100, Catalin Marinas wrote:
> > On Mon, 2011-05-09 at 11:32 +0100, Russell King - ARM Linux wrote:
> > > On Mon, May 09, 2011 at 11:22:19AM +0100, Catalin Marinas wrote:
> > > > Alternatively an exception return would do as well (like movs pc, lr)
> > > > but I think we still add some code for setting up the SPSR.
> > >
> > > That gives us a way out of both of these without introducing any CPU
> > > specific code.  We can setup the SPSR before this block of code, and
> > > call it with two movs pc, reg instructions which will provide the
> > > necessary synchronization.
> >
> > We still need an ISB before enabling the MMU to make sure that the TTBR
> > changing is visible. We may run with the MMU enabled (in the identity
> > mapping) before the exception return but with random data in TTBR.
> 
>   Changes to CP15 registers and the memory order model
>   All changes to CP15 registers that appear in program order after any
>   explicit memory operations are guaranteed not to affect those memory
>   operations.
> 
>   Any change to CP15 registers is guaranteed to be visible to subsequent
>   instructions only after one of:
>   • the execution of an ISB instruction
>   • the taking of an exception
>   • the return from an exception.
...
> So, my reading of this suggests that ISB and returning from an exception
> (iow, movs pc, reg) have the same properties.  So:
> 
>         mcr     p15, 0, r5, c3, c0, 0           @ load domain access register
>         mcr     p15, 0, r4, c2, c0, 0           @ load page table pointer
> -       b       __turn_mmu_on
> +       mrs     r4, cpsr                        @ copy cpsr to spsr
> +       msr     spsr, r4

This doesn't work. From the ARM ARM (B1.3.3):

        The execution state bits are the IT[7:0], J, E, and T bits. In
        exception modes you can read or write these bits in the current
        SPSR.
        In the CPSR, unless the processor is in Debug state:
        • The execution state bits, other than the E bit, are RAZ when
        read by an MRS instruction.

So reading the CPSR doesn't copy the T and E bits. Of course, we could
set them explicitly but I find the ISB much simpler (and in practice we
only need it for ARMv7 onwards but adding the ARMv6 in case we have a
kernel compiled for both).
 
Catalin


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