linux-kernel - Re: [RFC 1/2] x86/entry/64: Use the TSS sp2 slot for rsp

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Message-ID: <20180722201211.GA30923@gmail.com>
Date:   Sun, 22 Jul 2018 22:12:11 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RFC 1/2] x86/entry/64: Use the TSS sp2 slot for rsp_scratch


* Andy Lutomirski <luto@...nel.org> wrote:

> In the non-trampoline SYSCALL64 path, we use a percpu variable to
> temporarily store the user RSP value.  Instead of a separate
> variable, use the otherwise unused sp2 slot in the TSS.  This will
> improve cache locality, as the sp1 slot is already used in the same
> code to find the kernel stack.  It will also simplify a future
> change to make the non-trampoline path work in PTI mode.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/include/asm/processor.h   | 5 +++++
>  arch/x86/include/asm/thread_info.h | 1 +
>  arch/x86/kernel/asm-offsets_64.c   | 1 +
>  arch/x86/kernel/process_64.c       | 2 --
>  4 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> index cfd29ee8c3da..2ef4c39ded45 100644
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -308,7 +308,12 @@ struct x86_hw_tss {
>  	 */
>  	u64			sp1;
>  
> +	/*
> +	 * sp2 is scratch space used by the SYSCALL64 handler.  Linux does
> +	 * not use rung 2, so sp2 is not otherwise needed.
> +	 */
>  	u64			sp2;

s/rung/ring

Thanks,

	Ingo