linux-kernel - Re: [PATCH v5 02/19] ARM: LPAE: add ISBs around MMU enabling code

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Date:	Mon, 09 May 2011 16:38:50 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Will Deacon <Will.Deacon@....com>
Subject: Re: [PATCH v5 02/19] ARM: LPAE: add ISBs around MMU enabling code

On Mon, 2011-05-09 at 16:34 +0100, Russell King - ARM Linux wrote:
> On Mon, May 09, 2011 at 04:01:56PM +0100, Catalin Marinas wrote:
> > 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).
> 
> Err.  If that's correct then the Linux kernel is totally broken, and
> that's an incompatible change to the behaviour of the MRS and MSR
> instructions which has gone unnoticed.
> 
> We use "MRS reg, cpsr" for saving the IRQ state in SVC mode and
> "MSR cpsr, reg" to restore the interrupt state.  If the T bit gets
> reset by that, then Thumb kernels can never work.
> 
> What you've just said tells me that our implementation of:
> - arch_local_irq_save()
> - arch_local_save_flags()
> - arch_local_irq_restore()
> won't work because we can't read or write the I and F bits using
> MSR/MRS, even in SVC mode.

You can't write the execution state bits: IT[7:0], E and T.

You can write mask bits A, I and F using MSR.

> What is the replacement method for doing this?

For changing the execution state - SETEND, BX etc.

-- 
Catalin


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