linux-kernel - Re: [PATCH v4 2/4] objtool/powerpc: Add support for decoding all types of uncond branches

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Message-ID: <20241204140933.GV29862@gate.crashing.org>
Date: Wed, 4 Dec 2024 08:09:33 -0600
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Shrikanth Hegde <sshegde@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>, Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>, Naveen N Rao <naveen@...nel.org>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v4 2/4] objtool/powerpc: Add support for decoding all types of uncond branches

On Tue, Dec 03, 2024 at 08:44:50PM +0100, Christophe Leroy wrote:
> Add support for 'bla' instruction.
> 
> This is done by 'flagging' the address as an absolute address so that
> arch_jump_destination() can calculate it as expected. Because code is
> _always_ 4 bytes aligned, use bit 30 as flag.

The AA field already is there, so why not, eh :-)

> Also add support for 'b' and 'ba' instructions. Objtool call them jumps.

> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Reviewed-by: Segher Boessenkool <segher@...rnel.crashing.org>

> --- a/tools/objtool/arch/powerpc/decode.c
> +++ b/tools/objtool/arch/powerpc/decode.c
> @@ -55,12 +55,15 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
>  
>  	switch (opcode) {
>  	case 18: /* b[l][a] */
> -		if ((ins & 3) == 1) /* bl */
> +		if (ins & 1)	/* bl[a] */
>  			typ = INSN_CALL;
> +		else		/* b[a] */
> +			typ = INSN_JUMP_UNCONDITIONAL;
>  
>  		imm = ins & 0x3fffffc;
>  		if (imm & 0x2000000)
>  			imm -= 0x4000000;
> +		imm |= ins & 2;	/* AA flag */

You could of course put that together with the 3fffffc thing, but you
can leave that to the compiler as well :-)


Segher