linux-kernel - Re: [PATCH] powerpc/8xx: Fix clearing of bits 20-23 in ITLB miss

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Date:   Mon, 17 Feb 2020 11:10:09 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/8xx: Fix clearing of bits 20-23 in ITLB miss

On Sat, 2020-02-15 at 11:17 +0100, Christophe Leroy wrote:
> 
> Le 15/02/2020 à 07:28, Leonardo Bras a écrit :
> > On Sun, 2020-02-09 at 18:14 +0000, Christophe Leroy wrote:
> > > In ITLB miss handled the line supposed to clear bits 20-23 on the
> > > L2 ITLB entry is buggy and does indeed nothing, leading to undefined
> > > value which could allow execution when it shouldn't.
> > > 
> > > Properly do the clearing with the relevant instruction.
> > > 
> > > Fixes: 74fabcadfd43 ("powerpc/8xx: don't use r12/SPRN_SPRG_SCRATCH2 in TLB Miss handlers")
> > > Cc: stable@...r.kernel.org
> > > Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
> > > ---
> > >   arch/powerpc/kernel/head_8xx.S | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/powerpc/kernel/head_8xx.S b/arch/powerpc/kernel/head_8xx.S
> > > index 9922306ae512..073a651787df 100644
> > > --- a/arch/powerpc/kernel/head_8xx.S
> > > +++ b/arch/powerpc/kernel/head_8xx.S
> > > @@ -256,7 +256,7 @@ InstructionTLBMiss:
> > >   	 * set.  All other Linux PTE bits control the behavior
> > >   	 * of the MMU.
> > >   	 */
> > > -	rlwimi	r10, r10, 0, 0x0f00	/* Clear bits 20-23 */
> > > +	rlwinm	r10, r10, 0, ~0x0f00	/* Clear bits 20-23 */
> > >   	rlwimi	r10, r10, 4, 0x0400	/* Copy _PAGE_EXEC into bit 21 */
> > >   	ori	r10, r10, RPN_PATTERN | 0x200 /* Set 22 and 24-27 */
> > >   	mtspr	SPRN_MI_RPN, r10	/* Update TLB entry */
> > 
> > Looks a valid change.
> > rlwimi  r10, r10, 0, 0x0f00 means:
> > r10 = ((r10 << 0) & 0x0f00) | (r10 & ~0x0f00) which ends up being
> > r10 = r10
> > 
> > On ISA, rlwinm is recommended for clearing high order bits.
> > rlwinm  r10, r10, 0, ~0x0f00 means:
> > r10 = (r10 << 0) & ~0x0f00
> > 
> > Which does exactly what the comments suggests.
> > 
> > FWIW:
> > Reviwed-by: Leonardo Bras <leonardo@...ux.ibm.com>
> > 
> 
> I guess you mean
> 
> Reviewed-by: Leonardo Bras <leonardo@...ux.ibm.com>

Yes, sorry for the typo.

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