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

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Date:   Mon, 23 Jul 2018 19:36:42 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        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 Mon, Jul 23, 2018 at 5:38 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> 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.
>          */

I'll improve this for v2.