linux-kernel - Re: [PATCH] x86/mm/encrypt: Use %a asm operand modifier to obtain %rip-relative address

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Message-ID: <20231120171507.GGZVuUG9aSLyF52jHd@fat_crate.local>
Date:   Mon, 20 Nov 2023 18:15:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/mm/encrypt: Use %a asm operand modifier to obtain
 %rip-relative address

On Mon, Nov 20, 2023 at 04:33:50PM +0100, Uros Bizjak wrote:
> The "a" asm operand modifier substitutes a memory reference, with the
> actual operand treated as address.  For x86_64, when a symbol is
> provided, the "a" modifier emits "sym(%rip)" instead of "sym".
> 
> Clang allows only "i" and "r" operand constraints with an "a" modifier,
> so the patch normalizes the modifier/constraint pair to "a"/"i"

s/the patch normalizes/normalize/

> which is consistent between both compilers.
> 
> No functional change intended.
> 
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> ---
>  arch/x86/mm/mem_encrypt_identity.c | 16 ++++------------
>  1 file changed, 4 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
> index d73aeb16417f..6a351fdd1b39 100644
> --- a/arch/x86/mm/mem_encrypt_identity.c
> +++ b/arch/x86/mm/mem_encrypt_identity.c
> @@ -346,9 +346,7 @@ void __init sme_encrypt_kernel(struct boot_params *bp)
>  	 * We're running identity mapped, so we must obtain the address to the
>  	 * SME encryption workarea using rip-relative addressing.
>  	 */
> -	asm ("lea sme_workarea(%%rip), %0"
> -	     : "=r" (workarea_start)
> -	     : "p" (sme_workarea));
> +	asm ("lea %a1, %0" : "=r" (workarea_start) : "i" (sme_workarea));

Yeah, I saw that particular subthread today.

Are you sure this "a" modifier DTRT with all gcc version we support?

I.e., from 5.1 onwards...

Just making sure.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette