linux-kernel - Re: [PATCH] parisc/unaligned: Rewrite 64-bit inline assembly of emulate

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Date: Mon, 26 Feb 2024 11:29:17 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Helge Deller <deller@....de>
Cc: linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] parisc/unaligned: Rewrite 64-bit inline assembly of
 emulate_ldd()

Hi Helge,

On Thu, Feb 15, 2024 at 11:33:15PM -0800, Guenter Roeck wrote:
> Convert to use real temp variables instead of clobbering processor
> registers. This aligns the 64-bit inline assembly code with the 32-bit
> assembly code which was rewritten with commit 427c1073a2a1
> ("parisc/unaligned: Rewrite 32-bit inline assembly of emulate_ldd()").
> 
> While at it, fix comment in 32-bit rewrite code. Temporary variables are
> now used for both 32-bit and 64-bit code, so move their declarations
> to the function header.
> 
> No functional change intended.
> 
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---
> Implemented while analyzing a bug. I am not really sure of it is worth
> the effort, but I figured that I might as well submit it.
> 
>  arch/parisc/kernel/unaligned.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/parisc/kernel/unaligned.c b/arch/parisc/kernel/unaligned.c
> index c520e551a165..622c7b549fb8 100644
> --- a/arch/parisc/kernel/unaligned.c
> +++ b/arch/parisc/kernel/unaligned.c
> @@ -169,7 +169,8 @@ static int emulate_ldw(struct pt_regs *regs, int toreg, int flop)
>  static int emulate_ldd(struct pt_regs *regs, int toreg, int flop)
>  {
>  	unsigned long saddr = regs->ior;
> -	__u64 val = 0;
> +	unsigned long shift;
> +	__u64 val = 0, temp1;
>  	ASM_EXCEPTIONTABLE_VAR(ret);
>  
>  	DPRINTF("load " RFMT ":" RFMT " to r%d for 8 bytes\n", 
> @@ -180,25 +181,22 @@ static int emulate_ldd(struct pt_regs *regs, int toreg, int flop)
>  
>  #ifdef CONFIG_64BIT
>  	__asm__ __volatile__  (
> -"	depd,z	%3,60,3,%%r19\n"		/* r19=(ofs&7)*8 */
> -"	mtsp	%4, %%sr1\n"
> -"	depd	%%r0,63,3,%3\n"
> -"1:	ldd	0(%%sr1,%3),%0\n"
> -"2:	ldd	8(%%sr1,%3),%%r20\n"
> -"	subi	64,%%r19,%%r19\n"
> -"	mtsar	%%r19\n"
> -"	shrpd	%0,%%r20,%%sar,%0\n"
> +"	depd,z	%4,60,3,%2\n"		/* shift=(ofs&7)*8 */
> +"	mtsp	%5, %%sr1\n"
> +"	depd	%%r0,63,3,%4\n"
> +"1:	ldd	0(%%sr1,%4),%0\n"
> +"2:	ldd	8(%%sr1,%4),%3\n"
> +"	subi	64,%2,%2\n"
> +"	mtsar	%2\n"
> +"	shrpd	%0,%3,%%sar,%0\n"
>  "3:	\n"
>  	ASM_EXCEPTIONTABLE_ENTRY_EFAULT(1b, 3b, "%1")
>  	ASM_EXCEPTIONTABLE_ENTRY_EFAULT(2b, 3b, "%1")
> -	: "=r" (val), "+r" (ret)
> -	: "0" (val), "r" (saddr), "r" (regs->isr)
> -	: "r19", "r20" );
> +	: "+r" (val), "+r" (ret), "=&r" (shift), "=&r" (temp1)
> +	: "r" (saddr), "r" (regs->isr) );

It looks like something went wrong when this patch was applied. It is now

+"      depd,z  %4,60,3,%3\n"           /* shift=(ofs&7)*8 */
..
+       : "+r" (val), "+r" (ret), "+r" (saddr), "=&r" (shift), "=&r" (temp1)
+       : "r" (regs->isr) );

meaning saddr is now %2, but the depd,z instruction
still assumes it is %4. Unfortunately this results in a crash
when trying to boot linux-next on parisc64.

The patch below on top of linux-next fixes the problem for me.

Guenter

---
iff --git a/arch/parisc/kernel/unaligned.c b/arch/parisc/kernel/unaligned.c
index 31974eddedc9..a8e75e5b884a 100644
--- a/arch/parisc/kernel/unaligned.c
+++ b/arch/parisc/kernel/unaligned.c
@@ -181,7 +181,7 @@ static int emulate_ldd(struct pt_regs *regs, int toreg, int flop)

 #ifdef CONFIG_64BIT
        __asm__ __volatile__  (
-"      depd,z  %4,60,3,%3\n"           /* shift=(ofs&7)*8 */
+"      depd,z  %2,60,3,%3\n"           /* shift=(ofs&7)*8 */
 "      mtsp    %5, %%sr1\n"
 "      depd    %%r0,63,3,%2\n"
 "1:    ldd     0(%%sr1,%2),%0\n"