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

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Message-ID: <854ac759-efec-3e35-59a9-8da35b2b5156@linux.intel.com>
Date:   Mon, 23 Jul 2018 05:38:27 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC 1/2] x86/entry/64: Use the TSS sp2 slot for rsp_scratch

On 07/22/2018 10:45 AM, Andy Lutomirski wrote:
> +	/*
> +	 * sp2 is scratch space used by the SYSCALL64 handler.  Linux does
> +	 * not use rung 2, so sp2 is not otherwise needed.
> +	 */
>  	u64			sp2;

Could we call out the actual thing that we use this slot for, and the
symbol name so folks can find the corresponding code that does this?
While I know the spot in entry_64 you're talking about, it might not be
patently obvious to everyone, and it's also a bit more challenging to
grep for than normal C code.

Maybe:

	/*
	 * Since Linux does not use ring 2, the 'sp2' slot is unused.
	 * entry_SYSCALL_64 uses this as scratch space to stash the user
	 * %RSP value.
	 */